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The board wants twice the pipeline next year. Marketing gets asked the obvious question: if PPC brings in customers at £600 each, why not double the budget and double the customers? Anyone who has tried knows what happens next. Spend goes up, CPCs go up with it, lead quality drifts, and three months later CAC is 40% higher and nobody is quite sure why.
You can scale PPC spend by 100% or more and hold CAC where it is. You just can't always do it by pushing more money through the same campaigns. This post explains why CAC rises when budgets grow, and the specific changes we make to B2B and SaaS accounts so that the second £30,000 a month buys customers at roughly the same price as the first.
Why CAC rises when you scale PPC spend
Every campaign has a demand ceiling. There are only so many people searching "expense management software" in the UK each month, and your ads are already showing to most of the ones who convert well. When you add budget to a campaign that's near its ceiling, you don't find more of the same buyers. You pay more for the ones you already had, and you start buying the ones who were never going to convert.
The bidding algorithm does exactly what you tell it. Raise the budget or loosen a target CPA and Smart Bidding will bid higher on auctions it previously passed on, broaden into looser match queries and show more often in lower-intent placements. Each of those extra conversions costs more than the last.
That's the difference between average CAC and marginal CAC, and it's the number most scaling plans ignore. Here's an illustrative example:
- A non-brand Google Ads account spends £30,000 a month and brings in 50 new customers. CAC is £600.
- The budget doubles to £60,000 on the same campaigns. The extra £30,000 brings in 25 more customers, not 50.
- The marginal CAC on that second £30,000 is £1,200. Blended CAC is now £800, a third worse, and the dashboard only shows the blended figure.
So the goal isn't to keep every pound as efficient as the first. It's to find enough new, efficient spend that the blended number doesn't move.
Step 1: Measure CAC on revenue, not on form fills
Most accounts we audit can't scale safely because they're optimising towards the wrong conversion. If Google is bidding on form submissions, it will happily find you more form submissions as budget grows: students, job seekers, competitors and companies far too small to buy. Platform CPL stays flat while real CAC climbs.
Before you add a pound of budget, connect your CRM to the ad platforms. Offline conversion import lets you send Google back the leads that became sales-qualified, the ones that became opportunities and the ones that closed, with their values. Once bidding learns from pipeline rather than form fills, two useful things happen. The algorithm stops paying for leads sales won't touch, and you can see which campaigns, keywords and markets produce customers at all.
That clean-up is where scaling budget usually comes from. It's common to find a meaningful share of existing spend going on queries and placements that produce no qualified pipeline. Cutting it lowers CAC in the core account, and the efficiency you recover in the existing account is what pays for the learning curve in new ones. We cover the difference between platform-reported results and real outcomes in more depth in measurement vs attribution in PPC.
Step 2: Find your marginal CAC before you commit
Once you're measuring the right thing, work out where each campaign sits on its curve. Some have headroom. Others are already paying the premium.
Brand campaigns are the classic trap. They show a very low CAC because most of those people would have found you anyway. Report brand separately and leave it out of any scaling decision. Doubling brand spend rarely doubles brand demand.
For non-brand campaigns, increase budgets in steps of around 20% to 30% and hold each step long enough to read the result. Google says the learning period after a bid strategy change typically runs one to two conversion cycles, so in a B2B account where clicks take three weeks to become opportunities, a one-week test tells you very little. Compare the extra customers from each step against the extra spend. When the marginal CAC on a step lands well above your target, that campaign has found its ceiling and the next pound belongs somewhere else. Google's Performance Planner can give you a starting forecast, but your own step tests on CRM data are the version to trust.
Step 3: Scale sideways into new demand, not deeper into the same demand
This is the part that does most of the work. Instead of pushing one campaign up its cost curve, you add new pools of demand and run each one at the efficient end of its own curve.
The pools we use most often in B2B and SaaS accounts are:
- Microsoft Ads. Usually cheaper clicks and a desktop-heavy, corporate audience. Most of the Google build imports directly, so it's often the fastest efficient spend you can add.
- New keyword themes. Problem-aware searches ("how to reduce month-end close time"), competitor and "alternative to" terms, and integration terms ("HubSpot expense tool") each reach buyers your core product terms miss.
- New markets. Launching in the US, DACH or the Nordics opens up a new ceiling entirely. Each market needs its own campaigns and its own budget floor, as we explain in our guide to how much PPC costs for B2B and SaaS.
- New platforms. LinkedIn Ads for B2B reaches named accounts who aren't searching yet. ChatGPT Ads is newer, but for one UK SaaS client the CPC has averaged £1.96 against £36 on Google, with CAC at £200 against £600.
Not every pool will beat your current CAC, and that's fine. A new market running at £700 CAC is a good addition if the core account has dropped to £540 after the clean-up. What matters is the blended number across everything, measured on the same CRM definition of a customer.
Step 4: Raise conversion rate so the same clicks buy more customers
CAC is a simple chain: what you pay per click, divided by how many of those clicks become customers. Most scaling plans only work on the first half.
Say your average CPC is £30 and 1% of clicks become customers. CAC is £3,000. Lift that click-to-customer rate to 1.5% and CAC falls to £2,000, or you can afford a £45 CPC at the same £3,000 CAC. That extra 50% of bidding headroom is what lets you compete in auctions you were priced out of, which is exactly what scaling requires.
The biggest levers are usually outside the ad account. Landing pages that match the search intent (a comparison page for "alternative to" searches rather than the homepage), shorter forms for high-intent terms, demo booking straight from the page and faster sales follow-up all move this number. Conversion rate optimisation is often the cheapest way to create room to scale, because it improves every channel at once.
Step 5: Tell the algorithm which customers are worth more
A £5,000-a-year customer and a £60,000-a-year customer count as one conversion each in most accounts. That forces the bidding to value them equally, so it pays the same for both.
Pass deal size or predicted contract value back through your CRM and switch to value-based bidding. Google can then bid harder for searches that look like enterprise buyers and pull back on the ones that look like small accounts. For sales-led SaaS, this often matters more than any budget change. Your CAC might stay flat while the average contract value of the customers you acquire goes up, which is the version of "same CAC" your finance team will care about most.
Step 6: Scale in stages, with a rule for when to stop
Doubling spend in a month almost always goes badly. It resets learning across the account, floods the sales team with volume they can't qualify and makes it impossible to see which change caused which result.
A more reliable pattern is to plan the move over three to four months. Fix measurement and cut waste first. Add the pools with the most predictable returns next, usually Microsoft Ads and new keyword themes, then new markets and platforms. Set a marginal CAC ceiling for every new pool before it launches, and agree in advance what happens if it's breached after two conversion cycles: pull the budget back, fix the landing page, or shut it down. Review marginal CAC by pool every fortnight, not just the blended figure.
What a doubled account can look like
Here's an illustrative version of the £30,000-a-month account from earlier, after the changes above. Spend has doubled to £60,000. Brand is reported separately and excluded.
Half the new pools run above the original £600 CAC. The account still holds its number because the core got cheaper when bidding switched to pipeline data, and that saving funds the newer, less efficient spend while it learns. Push £60,000 through the original campaigns instead and you'd be closer to the £800 in the first example.
The question to ask of your own account, then, isn't whether it can take more budget. It's where the next pound would actually go, and what it would cost at the margin. If you don't have a clear answer, book a PPC audit and we'll show you your marginal CAC by campaign, how much of your current spend is producing pipeline and which new pools could take budget without pushing CAC up.
Last reviewed: September 2026 by Michéal Breslin, Managing Director
