I see posts in here almost every week from business owners furious that they doubled their Google Ads budget from $10k to $20k a month and watched their cost per lead go from $80 to $190. Everyone immediately blames the media buyer or assumes Google broke something in an update.
Sometimes the setup really is trash. Plenty of shops throw a client into a single unsegmented campaign and walk away.
But after spending years managing large scale paid search budgets in brutal, high CPC verticals, the actual culprit in mature accounts is usually the math of marginal return, and almost nobody talks to business owners about it honestly because agencies get paid on a percentage of ad spend.
When you spend your first $10k a month on search, Google's bidding algorithm cherry picks the lowest hanging fruit in the auction. It buys the users who are practically holding their credit card in their hand at 2 PM on a Tuesday. Your average CPA looks incredible at $80.
When you tell your agency to double the spend to $20k within the same geographic footprint and service line, you don't get twice as many $80 leads. The pool of ready to buy searches is finite. To spend that extra $10k, the algorithm is forced to bid higher into more competitive auctions, reach further up the funnel, and capture users with weaker intent.
So if your first 125 leads cost $80 each ($10k spend), and you force another $10k into the engine that only yields 40 additional leads, your blended CPA on the report says $121. That still looks somewhat tolerable on a monthly PDF. What the report hides is that the marginal CPA on that second $10k tranche of spend was actually $250 per lead. If your break even on a lead is $150, you literally lost money on every single dollar of the expansion, even though the overall account still looks profitable on a blended average.
Most agencies won't show you a marginal diminishing return curve because it talks them out of higher management fees.
Before you let anyone talk you into scaling spend or launching Performance Max to "find new pockets of inventory," force them to pull an impression share lost to rank vs budget report on your core exact match converting queries, and calculate the cost per acquisition strictly on the incremental dollars added over the last 60 days. If your marginal CPA is above your net contribution margin, scaling ad spend is just subsidizing Google's quarterly earnings at the expense of your bottom line.
Curious how many people here actually look at incremental tranche costs versus just looking at the blended monthly ROAS or CPA number on their dashboard.