Module 1: Thinking Like an Advanced PPC Practitioner
Lesson 3 · Module 1 · Thinking Like an Advanced PPC Practitioner
Lesson 3 objective
Understand why average CPA and ROAS can hide the economics of growth, and learn to evaluate the marginal performance of additional spend.
The Average Problem
Average performance can make growth look better than it really is.
Suppose an account spends £10,000 and generates 200 conversions. The average CPA is £50.
You then increase spend to £14,000 and generate 250 conversions. The new account-level CPA becomes £56.
It is tempting to say the extra budget produced conversions at roughly £56 each.
It did not.
Marginal CPA looks only at the additional spend and additional conversions.
Additional spend = £4,000
Additional conversions = 50
Marginal CPA = £4,000 ÷ 50 = £80
Additional conversions = 50
Marginal CPA = £4,000 ÷ 50 = £80
The account average is £56 CPA. The additional growth cost £80 per conversion.
Why This Happens
Scaling usually moves you further down the opportunity curve.
The cheapest, highest-intent opportunities are often captured first. As spend increases, Google may need to enter more expensive auctions, reach broader demand or accept lower predicted conversion probability to use the additional budget.
Higher auction costs
More aggressive participation may expose the account to progressively more expensive clicks.
Broader demand
Incremental volume may come from users with weaker purchase or enquiry intent.
Reduced efficiency
Conversion rate may decline as the account expands beyond its most efficient traffic.
Finite demand
There may simply be a limited volume of highly profitable searches available.
Worked Example
Account averages can conceal deteriorating marginal returns.
| Spend | Conversions | Average CPA | Extra Spend | Extra Conversions | Marginal CPA |
|---|---|---|---|---|---|
| £5,000 | 125 | £40 | — | — | — |
| £7,000 | 160 | £43.75 | £2,000 | 35 | £57.14 |
| £10,000 | 200 | £50 | £3,000 | 40 | £75 |
| £14,000 | 250 | £56 | £4,000 | 50 | £80 |
The account may still be worth scaling. The question is whether an £80 marginal CPA is commercially acceptable.
ROAS Works The Same Way
Average ROAS can hide weak incremental revenue.
If ecommerce spend rises from £20,000 to £25,000 while revenue rises from £100,000 to £112,000, the new average ROAS is 4.48.
But the additional £5,000 spend produced only £12,000 of additional revenue.
Marginal ROAS
Additional revenue = £12,000
Additional spend = £5,000
Marginal ROAS = 2.4
Additional spend = £5,000
Marginal ROAS = 2.4
If the business requires a ROAS of 3.0 to remain commercially attractive, the account average still looks healthy while the latest increment of spend does not.
Interactive Exercise
Calculate marginal CPA.
Compare two spend levels
Enter the earlier and later account performance.
Commercial Decision
A higher marginal CPA is not automatically bad.
This is one of the most important distinctions in advanced paid search.
If the business can profitably acquire customers at £90, then moving from an average CPA of £50 to a marginal CPA of £80 may still represent excellent growth.
Conversely, preserving a £50 average CPA by refusing to scale could leave profitable demand untapped.
The optimisation target is not “keep CPA as low as possible”. It is “capture profitable demand while understanding the cost of the next increment of growth”.
Practical Challenge
Analyse one real scaling decision.
- Choose two comparable periods with different spend levels.
- Calculate average CPA or ROAS for both periods.
- Calculate the additional spend.
- Calculate the additional conversions or revenue.
- Calculate marginal CPA or marginal ROAS.
- Compare the marginal result with the business's acceptable economics.
- Decide whether additional scaling was commercially justified.
Knowledge Check
Think marginally.
1. Spend increases from £8,000 to £10,000 and conversions increase from 160 to 190. What is the marginal CPA?
2. An account's average CPA rises from £45 to £52 after scaling, while the marginal CPA is £74. The business can profitably acquire customers up to £85. What is the best interpretation?
3. Why can average ROAS be misleading when evaluating additional spend?
4. What is the strongest principle for scaling an established Google Ads account?
Average performance tells you how the account has performed. Marginal performance tells you what the next stage of growth is costing.
Advanced PPC decisions need both.