Method 05 · Budget
Spend
curve.
Every extra dollar of ad spend buys a little less than the one before it. Your average cost per lead can look fine while the last few hundred dollars are being wasted. The Spend Curve finds the point where the next lead costs more than it's worth, so you know whether to spend more, hold, or spend less.
Written by Dan Picken, founder. Last checked 27 October 2026.The shape is typical. Where your stop point sits depends on what a lead is worth to you.
Averages
hide the waste.
Most reports show an average: total spend divided by total leads. But the first dollars buy the easiest, best-matched searches. The last dollars buy the leftovers.
For example, an account can average $60 a lead while the last $1,000 is buying leads at $150. The average never tells you.
Google's own bid and budget simulators show the same shape: more budget, fewer extra results per dollar.
What Google's
own tools show.
Google's simulators estimate what your results could have been over the previous 7 days at different bids or budgets. They're useful, and they show the curve, but they look back a week and assume nothing else changes.
They also go missing exactly when you'd want them. Google says a simulator may not be available if the campaign hit or nearly hit its daily budget in the last 7 days, or if it uses a shared budget.
Two other numbers help. Performance Planner forecasts how changes to your spend might affect results, looking forward rather than back, though it is still Google's model of the auction, not your sales. And Search lost impression share (budget) is the share of time your ads weren't shown because the budget ran out. If it is near zero, budget isn't what's holding you back, and more money can only go on higher bids or broader searches.
So the method combines three views: your own history at different spend levels, the simulators where they exist, and deliberate budget step tests when the decision is big enough to need proof.
How it
works.
- 1
Know what a lead is worth
From your close rate and the value of a sale. Without this, no budget is right or wrong.
- 2
Plot the curve
From your account's history and Google's simulators: results at each level of spend.
- 3
Find the stop point
Where the cost of the next lead crosses what a lead is worth to you.
- 4
Move the money
Spend past the stop point moves to a campaign, channel or month where it still pays.
Try it
What did the
last dollar buy?
Enter spend and conversions for one campaign across a few months (or weeks) at different budgets. It works out what each extra conversion cost as spend went up. The starting numbers are an example. Use your own.
Months differ for other reasons too: season, offers, competitors. Treat this as a first look. The full method tests budget steps on purpose so the comparison is fair.
| Spend ($) | Conversions |
|---|---|
Cost of the extra conversions
| Step | Average cost | Extra cost each |
|---|
The average cost per conversion often looks fine while the extra ones get expensive. That gap is the point of the Spend Curve.
In my experience
The conversation nobody expects: sometimes the right move is to spend less. I've looked at accounts where the last chunk of budget was buying the most expensive, least likely leads, and trimming it barely changed the lead count. Clients are usually more surprised by that than by any 'spend more' recommendation.
When it's
not worth it.
- Short histories mislead. A busy season can look like a budget effect.
- Automated bidding behaves differently for a while after a budget change, so each step needs time to settle.
- If tracking counts the wrong things, the curve has the wrong shape. Fix tracking first.
What you get
- What a lead or sale is worth to you
- The curve for each main campaign
- The stop point, in dollars a month
- Where to move spend that is past it
The technical bit
- Marginal cost
- What the next lead costs, not the average lead.
- Diminishing returns
- Each extra dollar buys a little less than the last.
- Break-even
- The point where a lead costs exactly what it is worth.
- Impression share
- How often your ad showed out of all the times it could have.
Where it's used
Questions
people ask.
Can I try it on my own numbers?
Yes. The interactive Spend Curve on the Google Ads page lets you drag the budget and the value of a lead and see where the stop point lands.
Does this mean I should cut my budget?
Not always. Some accounts are well short of their stop point and should spend more. The curve tells you which way to move, and roughly how far.
What is marginal cost per acquisition?
What the next conversion costs, rather than the average. If spend goes from $4,000 to $5,000 and conversions go from 60 to 64, those four extra conversions cost $250 each, even if the average is still about $78.
How do I find my stop point without much data?
Change the budget in steps, a few weeks each, and compare the extra conversions with the extra spend. The table on this page does the maths for you.
Why did my cost per lead go up when I raised the budget?
Usually because the extra money bought the leftovers: broader searches, weaker placements, less likely buyers. That's the curve doing what curves do.
The other methods
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