The short answer
Cost per lead = ad spend ÷ leads. Cost per acquisition (CPA) = ad spend ÷ customers. Your maximum affordable CPA is the gross profit from a customer minus the profit you want to keep. Multiply that by your close rate to get your maximum cost per lead.
Cost per lead vs cost per acquisition
Cost per lead (CPL) counts every enquiry, call or form. Cost per acquisition (CPA) counts only the ones that become customers. A campaign with a cheap CPL can have an expensive CPA if the leads don't close, which is why we always try to track both.
If you can only see CPL in your ad account, connect your CRM so closed deals flow back to Google, Microsoft, Meta or LinkedIn. It changes what the platforms optimise for.
Working out your maximum CPA
Start with what a customer is worth: the first sale, or the lifetime value if they come back. Multiply by your gross margin to get gross profit per customer. Decide how much of that you want to keep. What's left is the most you can spend to win a customer.
For example: a customer worth $3,000 at a 50% margin brings $1,500 gross profit. Keep 30% of that ($450) and you can spend up to $1,050 per customer. At a 20% close rate, that's a maximum of $210 per lead.