Why does adding budget always seem to raise our cost per job?
Because you buy the cheapest demand first. Your average cost per job blends the strong demand you were already capturing with the weaker demand your last budget increase bought. The number that matters for a budget decision is marginal cost per job — extra spend divided by the extra jobs it produced — and it is almost always worse than the average, often by a lot.
Average and marginal are different questions
Average cost per job asks what all your spend produced. Marginal cost per job asks what the last increment of spend produced. Only the second one answers whether to spend more, and it is the one almost nobody calculates.
The formula: take the change in spend between two comparable periods and divide it by the change in booked jobs. If spend rose and jobs barely moved, the marginal cost is enormous even though the average barely budged — because the average is anchored by demand you would have captured anyway.
Why the curve bends
- Intent thins out. The first budget captures people searching for exactly what you sell. Additional budget reaches broader queries and colder audiences.
- Geography stretches. Expanding radius adds drive time and lowers close rates before it lowers anything else.
- Auction pressure rises. Buying more of a fixed pool of high-intent impressions means paying above the previous clearing price.
- Your own capacity saturates. Extra leads that nobody answers or nobody can schedule convert at zero, which shows up as a marketing problem and is not one.
How to actually measure it
Change budget in one channel, in one step, large enough to exceed normal week-to-week variation, and hold everything else still. Wait a full lead-to-cash cycle. Then compare cohorted booked jobs before and after. Small continuous adjustments produce data that cannot be read, because every effect overlaps the next change.
Seasonality will confound a simple before-and-after, so compare against the same period in prior years or against a geography you did not change. Our reporting convention is to flag any spend change and hold the comparison window open until the cohort matures.
The decision rule
Keep spending while the marginal cost per booked job stays under what that job type can afford. Stop when it crosses. That is the whole rule, and it is different from the rule most teams use, which is to keep spending while the average still looks acceptable.
Applied consistently, it usually means some channels get more budget than instinct suggests and others get frozen well below their apparent efficiency. That reallocation is where most of the gain in home services marketing actually comes from.
Topics: marginal cost · diminishing returns · budget · paid media
Have a version of this question about your own business?
The useful answer usually depends on which systems you run and how they're connected. That's a conversation, not a blog post.