Break-even advertising for Amazon sellers: CPA and ROAS explained
Indicative trading calculations, not VAT or accounting advice. Check VAT treatment and figures with a qualified accountant before making pricing decisions.
Why traffic costs money
On a large marketplace, shoppers mostly see what ranks or what's paid for. New listings often have few reviews and little sales history, so sellers buy visibility with pay-per-click ads. You pay for clicks, not sales — and only a fraction of clicks buy. The total ad spend divided by the orders it produced is your real cost per order.
The key terms
- CPA (cost per acquisition)
- Ad spend ÷ orders from ads. E.g. £120 spent, 12 orders → £10 CPA.
- Break-even CPA
- Your contribution per order before ads. Spend exactly this per order and you make £0; more, and each sale loses money.
- ROAS (return on ad spend)
- Ad-driven sales ÷ ad spend. £600 of sales from £120 of ads = 5.0×.
- Break-even ROAS
- Price paid ÷ break-even CPA. Below this ROAS, ads lose money.
- ACoS
- Amazon's ratio: ad spend ÷ ad sales, as a percentage — the inverse of ROAS (5.0× ROAS = 20% ACoS).
Worked example: £59 product, VAT registered
Hypothetical assumptions: price £59 inc VAT; seller is VAT registered at 20%; product cost £20; fulfilment and delivery £4.50; packaging 50p; a referral fee of 15% of the price; returns averaging £1.50 per order. Costs are net of reclaimable VAT.
| Line | £ per order |
|---|---|
| Customer pays (inc VAT) | £59.00 |
| Output VAT owed to HMRC (£59 ÷ 1.2 × 0.2) | − £9.83 |
| Net revenue | £49.17 |
| Product cost | − £20.00 |
| Fulfilment / delivery | − £4.50 |
| Packaging | − £0.50 |
| Referral fee (15% of £59) | − £8.85 |
| Returns allowance | − £1.50 |
| Break-even CPA | £13.82 |
So each order can absorb up to £13.82 of advertising. Break-even ROAS is £59 ÷ £13.82 = 4.27×. If your campaign reports a ROAS below that, it is losing money on every ad-driven order. In Amazon's ACoS terms the same ceiling is £13.82 ÷ £59 = 23.4% break-even ACoS — measured against the gross, VAT-inclusive price, like Amazon's own ACoS. It ignores monthly overheads, so a campaign running exactly at it covers none of them.
Why VAT matters here
Ad platforms report ROAS on the price shoppers pay, which includes VAT. But £9.83 of each £59 is not yours. Ignore that and you'd think you could afford nearly £10 more per order than you can.
What different ad costs do to each order
| CPA | ROAS | Contribution | Margin |
|---|---|---|---|
| £0.00 | — | £13.82 | 28.1% |
| £5.00 | 11.80× | £8.82 | 17.9% |
| £10.00 | 5.90× | £3.82 | 7.8% |
| £13.82 (break-even) | 4.27× | £0.00 | 0.0% |
| £15.00 | 3.93× | -£1.18 | -2.4% |
Break-even is a ceiling, not a target. At break-even every sale contributes nothing to overheads or your pay. Many sellers set a target CPA well below it to leave a planned profit, and accept higher CPAs only deliberately (for example, during a launch where repeat purchases are expected — an assumption you should test, not assume).
Put your own price, fees and ad spend into the calculator — it shows break-even CPA and ROAS as you type.
Try your own numbersSelling through FBA? The Amazon FBA profit calculator adds referral and fulfilment fees, Individual vs Professional plans, storage and ROI.
Practical checks
- Calculate CPA from orders attributed to ads, not total orders, unless you deliberately want a blended view.
- Recheck break-even whenever fees, product cost, price or discounts change.
- Price promotions reduce break-even CPA — your bids may need to fall during a sale.
- Storage, long-term storage and other account-level fees aren't per-order; treat them as monthly overhead.
Limitations
Ad attribution is imperfect, and this model treats each order on its own without repeat purchases or halo effects. It's an arithmetic aid, not advertising, tax or accounting advice. For pricing basics see margin vs markup.