Three supplier quotes, one BOM: the line-by-line comparison that decided an M&E package

Steve Parker
Jul 4
8 min read

Updated: Jul 9

An Auckland electrical subbie held three supplier quotes for one commercial fit-out package, and the sharpest headline total was not the cheapest supplier. A line-by-line comparison on the overlapping scope put the incumbent wholesaler about $15–18k ahead — caught at tender, that difference went into the price instead of coming out of the margin mid-job.

By Steve Parker · Trueworks · NZ construction estimation · 7 min

What you'll learn in this case study

Why headline totals on supplier quotes mean almost nothing until every document is normalised to one bill of materials
How an overlap-only comparison separates genuine price advantage from scope that simply has not been priced
What a scope-gap register does with the lines only one supplier priced — and why those lines are risk, not savings

Quick answer: An Auckland electrical subcontractor pricing the M&E package for a commercial fit-out — a boutique studio tenancy — held three supplier quote documents: two electrical wholesalers, one of which had issued an original quote and then a revision. We normalised all three documents to a single line-item bill of materials, compared only the lines every supplier had priced, and treated everything else as scope risk. On the overlapping scope, the incumbent wholesaler came out about $15–18k cheaper. The alternative that looked sharper at the headline was missing lines that would have returned as extras mid-job, at whatever counter rate applied on the day. We recommended staying with the incumbent and documented the comparison in writing. Caught at tender, the gap became part of the price; caught mid-job, it would have come straight out of the margin.

The tender

An Auckland electrical subcontractor asked us to review supplier pricing before committing an M&E package tender for a commercial fit-out — a boutique studio tenancy in a mixed-use building. The package was supply-heavy: switchboard components, cable, containment, luminaires, accessories, and the long tail of small fittings that makes up any electrical bill of materials. Supply was going to be a low-to-mid six-figure component of the tender, so the supplier decision carried more weight than any single labour assumption in the job.

Three quote documents were on the desk. Two electrical wholesalers had priced the package. The incumbent — the wholesaler the subbie had traded with for years — had issued an original quote and then a revision after a drawing update. The alternative wholesaler had issued a single document with a headline total that looked meaningfully sharper. On totals alone, the alternative seemed the obvious pick, and the subbie's first instinct was to switch suppliers and bank the difference into a keener tender.

That instinct is exactly what a structured comparison exists to test. Most NZ commercial tenders run under NZS 3910 conventions, and the head contract will never care which wholesaler you chose — but the tender total you sign is yours to hold. The supply component of that total needs to reflect the whole scope, not the shortest document.

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What we found

The first job was mechanical: normalise all three documents to one line-item bill of materials. Wholesaler quotes rarely line up as issued — one prices cable by the metre, another by the drum; part numbers differ between merchants for equivalent product; descriptions are abbreviated differently; pack quantities vary. Until every document is expressed against the same BOM lines in the same units, any comparison of totals is a comparison of noise.

Once normalised, we compared only the overlapping lines — the items both wholesalers had actually priced. On that overlapping scope, the incumbent was about $15–18k cheaper. The alternative's sharper headline was built on a shorter document: sections of containment, several switchboard ancillaries, and a run of accessories were thin or absent. The headline total was lower because less of the job was in it.

The non-overlapping lines went into a scope-gap register rather than the comparison. This is the discipline that matters most: a line that only one supplier priced is not a price advantage for the supplier who left it out. It is scope risk. Every unpriced item returns mid-job as an extra, at the counter rate on the day it is needed, often with urgent delivery on top — and by then the tender margin is already committed.

We also compared the incumbent's original quote against its revision, line by line. Most movement traced cleanly to the drawing update — quantities shifted where the layout had changed — but a small number of unit rates had moved as well. The changes were modest, but rates can move under cover of a quantity change, and only a line-level check separates the two.

Our recommendation, documented in writing with the comparison table attached: stay with the incumbent. The subbie carried the right supply number into the tender and knew exactly which lines remained unpriced by anyone.

A second engagement — an industrial fit-out for the same trade — repeated the three-way comparison. The spread on overlapping scope was narrower there, and the subbie chose a sole supplier for tender cohesion: one supply document, one revision chain, one point of accountability if delivery slipped.

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How to run an overlap-only comparison yourself

The method is not complicated; it is just tedious, which is why it gets skipped under tender pressure.

Normalise first. Build one bill of materials in one set of units. Map every line of every supplier document onto it. Where a supplier prices by drum, carton, or pack, convert to the BOM unit before comparing anything.

Compare only the overlap. Sum each supplier across the lines that all documents priced. That subtotal — not the headline — is the like-for-like price signal.

Register the gaps. Every line that appears in fewer than all documents goes into a scope-gap register with a rough value against it. Ask the short-document supplier to price the missing lines before you decide anything; the answer is often revealing.

Check revisions line by line. When a supplier reissues a quote, difference the two documents. Confirm that rate changes are deliberate and explained, not riding along with quantity changes.

Document the decision. One page: overlap subtotals, gap register, recommendation, date. If the supplier question ever comes up post-award — from the main contractor, the client's quantity surveyor, or your own accountant — the answer is already in writing.

For a package of this size, the whole exercise took roughly a day. Against a $15–18k swing, that is the cheapest day on the job.

What it costs when it's caught late

| Stage caught | Cost range | Why | |---|---|---| | At tender | ~$1–2k | A day of comparison work; the gap is priced before anything is committed | | Post-award, before ordering | ~$5–10k | Renegotiating supply under programme pressure, with the tender total already fixed | | On site, mid-job | ~$15–25k | Unpriced lines return as extras at counter rates, plus urgent delivery premiums and lost labour productivity while materials are chased | | At final account | ~$20–40k | Supply overruns absorbed into the margin because no variation entitlement exists for your own procurement gap | | In dispute | ~$30–60k+ | Professional fees and management time arguing over scope that was never priced by anyone, with thin records |

Five checks before you commit to a supplier

Normalise every quote to one BOM in one set of units — never compare documents as issued.
Compare overlap-only subtotals, and treat the headline totals as marketing until proven otherwise.
Build a scope-gap register of every line that fewer than all suppliers priced, with an approximate value on each.
Difference every revision against its original at line level, and query rate movements that lack an explanation.
Record the decision in writing — overlap subtotals, gaps, recommendation — and file it with the tender.

FAQ — supplier quote comparison

Q1: Isn't the lowest headline total usually the cheapest supplier? Not reliably. A lower headline often means a shorter document. In this case the sharper-looking quote was missing whole sections of containment and ancillaries; on the scope both wholesalers actually priced, the higher-headline supplier was about $15–18k cheaper. The overlap subtotal is the honest number.

Q2: How long does a three-way comparison take? For a supply package in the low-to-mid six figures, roughly a day: a few hours to normalise the documents to one BOM, a few hours to match lines and build the gap register. It compresses further once you have a normalisation template for your usual merchants.

Q3: What should we do with lines only one supplier priced? Treat them as risk, not savings. Ask the other supplier to price them before deciding. If they decline or the answer is vague, assume those lines return mid-job as extras at spot rates and weight the comparison accordingly.

Q4: Should we split the package across suppliers to get the best of each? Sometimes, but be deliberate. Splitting adds a second revision chain, a second delivery stream, and a second point of failure. On the industrial fit-out engagement, the subbie chose a sole supplier for exactly that reason — tender cohesion was worth more than a marginal saving on a subset of lines.

Q5: Why compare a revised quote against its original at all? Because a reissue is the easiest place for unit rates to move unnoticed. Most revisions are honest responses to drawing changes, but the only way to know is to difference the documents line by line and ask about anything that moved without a quantity reason.

Who this helps

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About Trueworks

Trueworks is built by Steve Parker — 20 years on the analytical side of NZ construction. Variation reviews, contract advisory, programme review, and document-heavy estimation work. Trueworks is the productisation of that practice for NZ trades and builders: the same defensible analysis, at a price and pace a working contractor can actually use.

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