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CPA is what you paid in ads for each conversion, so $5,000 of spend that produced 50 sign-ups is a $100 CPA. The number that matters more is the CPA you can afford, which is the profit one conversion brings you. When my wife's travel business paid $7 a lead and closed 1 in 6, each customer cost $42 against a profit above $750, and that gap is the whole game.
Your numbers
Your margin on one sale, or profit per customer times close rate for a lead.
What they mean
Your CPA is $100.00. Half of a poor CPA is the landing page.
Headroom is the number to watch. If your CPA is $100 and one conversion earns you $250, you have $150 to play with on every sale, and that is room to bid higher, widen your match types or open a second campaign.
When the headroom goes negative the account loses money on every conversion it wins. That happens quietly, because Google Ads shows a falling CPA as good news and never asks what a conversion was worth to you.
Profit per conversion is the input to get right. For a lead, multiply your close rate by the profit on one customer. A lead that closes 1 in 6 at $750 of profit is worth $125, so a $42 CPA leaves $83 of headroom on every single lead.
Set your target CPA below the maximum, because the maximum is break-even and a bad week will find it. I leave at least a third of the headroom untouched. My B2B PPC guide shows how a longer sales cycle changes that sum, and my PPC budget guide shows how many conversions a budget can buy.
A good CPA is anything below the profit one conversion brings you, with room to spare. There is no industry figure that beats your own margin. A $100 CPA is excellent for a $2,000 sale and a disaster for a $60 one.
Take the profit on one customer. If the conversion is a lead rather than a sale, multiply that profit by your close rate. The result is your break-even CPA, and I would aim to run at about two thirds of it.
Cost per lead counts the form fill or the phone call. Cost per acquisition counts a paying customer. The gap between them is your close rate, which the ad account cannot see, so a cheap lead can still be an expensive customer.
Usually one of three things. Competition pushed the cost per click up, the landing page stopped converting, or the campaign widened into searches that never buy. Check the conversion rate first, because that is the one you can fix fastest.