Manual CPC vs target-CPA bidding: when handing the algo the wheel cost me margin
The setup: Scaling a stable Facebook campaign, $200/day, +35% ROI on manual lowest-cost bidding. The temptation: switch to cost-cap to protect margin while scaling.
The move: Cloned the campaign, set cost cap at our break-even CPA, scaled both to $400/day over a week.
The numbers (illustrative): Manual campaign scaled to $400/day, ROI compressed to +22% (normal at scale). Cost-cap campaign refused to spend — delivered only $140/day because the cap was too tight, then when we loosened it, it overshot and hit +4% ROI as the algo chased volume.
The lesson: Bid caps protect you from the downside and rob you of the upside. The algo optimizes to your cap, not your profit. On a campaign that's already winning, manual scaling preserves more margin than handing control to a CPA target.
What I'd do differently: Cost caps belong on NEW campaigns where you're scared of overspend, not on proven winners. I bolted a safety belt onto a car that was already driving fine and slowed it down. Use caps to learn, manual to milk.
Arb Files
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