Media Buying Unit Economics: 4 Metrics That Matter

Running ads without knowing your key numbers is like driving blindfolded. These four metrics will tell you what’s working, what’s not, and where to scale. Let’s break them down with real-world context.

What Is CPP (Cost Per Purchase) In Media Buying?

CPP (cost per purchase) is the amount you spend on ads to generate one purchase. The formula is simple:

CPP = ad spend ÷ number of purchases

Spend £1,000 and get 50 purchases, and your CPP is £20. It’s the single most useful number for judging whether a paid campaign is actually pulling its weight, because it ties spend directly to the outcome that pays your wages: a sale.

One point of confusion worth clearing up early. In traditional media buying (TV and radio), CPP usually means cost per point — the cost of reaching one rating point, or 1% of your target audience, calculated as gross media cost ÷ gross rating points (GRPs). In digital and performance media buying, and throughout this article, CPP means cost per purchase. If someone’s quoting CPP against a broadcast schedule, they almost certainly mean cost per point. If they’re quoting it against a Meta or Google campaign, they mean cost per purchase.

How CPP differs from CPA and CAC:

• CPP vs CPA: Cost per acquisition (CPA) counts any conversion you’ve told the platform to optimise for — a sign-up, a lead, an add-to-cart, a download. CPP counts only completed purchases. So CPP is a stricter, revenue-tied version of CPA. If your campaign optimises for purchases, your CPA and CPP will be the same. If it optimises for leads, they won’t be.
• CPP vs CAC: Customer acquisition cost (CAC) is business-wide and strategic. It divides total sales and marketing cost (ad spend plus salaries, tools and overheads) by all new customers across every channel over a period. CPP is tactical and channel-specific — it only counts the ad spend behind purchases from one campaign or platform. Your CAC is almost always higher than your paid CPP, because CAC carries the costs CPP ignores.

Worked CPP Example (Using The £100 / £40 Product Above)

Take the same product from the CAC section below: it sells for £100, costs £40 to make, and after fulfilment, returns and overheads you’re left with £30 of margin per sale. That £30 is your CAC ceiling — the most you can pay to acquire a customer and still break even on the first order. Framed as CPP:

• Breakeven CPP: £30. At this cost per purchase you make nothing on the first sale, but you don’t lose anything either. Everything past this relies on repeat purchases.
• Target CPP: below £30. To bank a first-order profit, keep CPP under the ceiling. A CPP of £20 leaves you £10 of contribution margin on every sale.
• Max CPP (with repeat buyers): above £30. Only justified if you know your repeat rate and lifetime value can recover the loss. If a customer’s LTV is £240, you can comfortably run a CPP well above £30 and still come out ahead over time.

The rule of thumb: your healthy CPP is a function of margin and LTV, not of what other advertisers are paying. A “good” CPP is one that sits below your CAC ceiling, or above it only when your repeat revenue clearly covers the gap.

1. What’s Your Real CAC Limit Based on Margins?

Why it matters: If your margins can’t support your CAC, scaling ads will just burn cash.

Example:
You sell a product for £100. Your cost of goods is £40.
That leaves £60. Out of that, you pay:

• £10 for fulfilment
• £5 for returns
• £15 for overheads

Now you’ve got £30 left. That’s your max CAC if you want to break even on the first purchase. Spend more than that, and you’re relying on repeat sales to stay afloat.

Takeaway: Work backwards from your margins, not forwards from what others are paying.

2. How Does LTV Change Across Products?

Why it matters: Not all customers are worth the same over time.

Example:

• Customer A buys a £30 gift set once.
• Customer B buys a £20 moisturiser every month.

You might lose money acquiring both, but Customer B brings in £240 over a year. That’s your growth engine.

Takeaway: Track repeat purchases by product. Invest in the ones that lead to long-term value.

3. What’s Your Contribution Margin – And Why It Matters for Channels?

Why it matters: It’s what’s left after product costs and CAC – your actual profit per sale.

Example:
£100 sale

• £40 product cost
• £30 CAC
  = £30 contribution margin

If you spend £50 on CAC instead, you’re down to £10. Not much room left to operate.

Takeaway: Use contribution margin to decide where to scale. It’s the most honest view of return.

4. Are You Driving Incremental Revenue – Or Just Winning the Last Click?

Why it matters: Some channels make the sale. Others set up the sale. Don’t kill top-of-funnel efforts just because they don’t close.

Example:
You run a Meta campaign that warms people up. They Google your brand and convert. Google Ads gets the credit, but Meta did the heavy lifting.

Takeaway: Test holdouts, check blended ROAS, and look at full-funnel behaviour. Invest in what grows the pie, not what just grabs the last slice.

CPP vs CPA vs CAC vs ROAS: Which Metric To Use When

These terms get used interchangeably, and that’s where a lot of media buyers come unstuck. Each answers a different question, so the trick is knowing which one to reach for. Here’s the quick reference:

Metric	What it measures	Formula	When to use it
CPP (cost per purchase)	Ad cost to generate one purchase	Ad spend ÷ purchases	Judging whether a paid campaign is profitable per sale
CPA (cost per acquisition)	Ad cost per conversion of any type	Ad spend ÷ conversions	Optimising campaigns whose goal is a lead, sign-up or non-purchase action
CAC (customer acquisition cost)	Total cost to win a new customer, all-in	(Sales + marketing costs) ÷ new customers	Strategic budgeting and checking the business is viable across all channels
Contribution margin	Profit left after product costs and CAC	Revenue − variable costs − CAC	Deciding which products or channels to scale
ROAS (return on ad spend)	Revenue returned per £1 of ad spend	Ad revenue ÷ ad spend	Comparing channel efficiency and setting bid targets

The short version: use CPP to check a sale is worth making, CPA when the goal isn’t a purchase, CAC to sanity-check the whole business, contribution margin to decide where to scale, and ROAS to compare channels at a glance.

How To Lower Your CPP Without Killing Volume

Once you’ve defined your CPP, the obvious next question is how to bring it down without choking off sales. The levers, roughly in order of impact:

• Creative testing. Ad creative is usually the single biggest driver of performance. Test new angles, hooks and formats continuously — a fresh winner can move CPP more than any bid tweak.
• Audiences and exclusions. Cut wasted spend by excluding recent purchasers and poor-performing placements, and lean into the audiences that actually convert.
• Bid strategy. Match your bid strategy to your goal. A target-CPA or target-ROAS strategy set against your real CAC ceiling stops the platform chasing cheap-but-worthless conversions.
• Landing-page conversion rate. More purchases from the same clicks lowers CPP directly. Faster load times, clearer offers and a tighter checkout often beat any change made inside the ad account.

FAQ

What is CPP in media buying?
CPP stands for cost per purchase — the ad spend needed to generate one purchase, calculated as ad spend ÷ number of purchases. Note that in traditional TV and radio buying, CPP instead means cost per point (the cost of reaching 1% of a target audience). In digital and performance media buying it means cost per purchase.

What’s the difference between CPP and CPA?
CPA (cost per acquisition) counts any conversion you optimise for — a lead, sign-up, download or purchase. CPP counts only completed purchases. CPP is effectively a stricter, revenue-tied version of CPA; the two are identical only when a campaign optimises specifically for purchases.

How do you calculate cost per purchase?
Divide total ad spend by the number of purchases that spend generated. For example, £1,000 in ad spend producing 50 purchases gives a CPP of £20.

What is a good CPP?
A good CPP is one that sits below your CAC ceiling — the margin left after product, fulfilment, returns and overhead costs. It should never be set by copying what other advertisers pay. If your CPP is above the ceiling, it’s only healthy when repeat purchases and lifetime value clearly recover the difference.

Rounding Up

Before you scale your spend, know these four numbers inside and out:

• Your true CAC ceiling
• LTV by product or customer type
• Contribution margin per channel
• Which touchpoints drive net-new growth