About 9%, with spend and traffic unchanged. Acquisition cost is a division: ad budget over the number of customers won. If the same traffic produces 10% more orders, each customer costs 1 ÷ 1.1 of what they did before. The open question is who pockets the difference, and it's not always you.
The benchmark
| Conversion lift | Acquisition cost drop |
|---|---|
| +5% | -4.8% |
| +10% | -9.1% |
| +20% | -16.7% |
| +50% | -33.3% |
The relationship isn't symmetrical. To cut your acquisition cost by 20%, you need 25% more conversion. To cut it in half, you need to double the conversion rate of the pages that receive the ad traffic.
The rule
This math applies to the traffic you pay for, on the pages where it lands. When conversion improves among your loyal customers, on email, or on direct traffic, revenue goes up, but acquisition cost doesn't move, because those visits cost you nothing in advertising.
The authors learned this from their own numbers. In the review of a first year with a fashion brand they work with, conversion on email visits had gained about a quarter and direct traffic had improved a little. On social, where most of the ad budget went, it had slipped slightly.
The site converted better for people who already knew the brand. On paid traffic, nothing had moved in the right direction, and acquisition cost stayed where it was.
The steps
- The starting point. Divide the month's ad spend by the number of first orders recorded over the period, by campaign type when you can. Be wary of the paid conversion rate shown by Shopify or GA4 under last-click attribution: in an audit this year, a brand's entire paid traffic came out under 1% conversion, retargeting included, because sales triggered by an ad were credited to direct traffic or to branded search.
- The landing pages. In your analytics tool, find the pages that receive this traffic by filtering on paid sources.
- The work. Improve these pages before the rest of the site, even when they aren't your favorites.
- The new measurement. Recalculate acquisition cost on the same spend and the same time period, then compare the change to the benchmark above.
The trap
The instinct is to watch for the drop in the ad platform's dashboard, where it often doesn't show up. With a target-cost bid strategy, such as target CPA on Google or the cost per result goal on Meta, the platform tries to get as many conversions as possible around the cost you gave it, and it sets its bids based on the conversion probability it estimates.
A page that converts better therefore gives it room to bid higher and go after colder audiences.
In our reading, as long as you don't touch the target, the algorithm spends your conversion gain first: cost per customer stays stuck at what you asked for, and volume is what goes up.
That may suit you just fine, as long as you chose it on purpose. If you want to pay less per customer, lower the target as conversion improves.
The math to present
Take a hypothetical example. A budget of €10,000 a month brings in 200 first orders, which is an acquisition cost of €50.
If the landing pages gain 10% in conversion, the same budget brings in 220, and each customer costs about €45.50. Before you launch a single test, you can put the question to your leadership with this sentence:
"If we gain 10% conversion on the pages that receive ad traffic, we can either lower our target CPA and pay about €4.50 less per customer, or keep the target and buy about twenty more customers a month. We need to choose now, or the ad platform will choose for us."
