Plan a campaign before you spend
Divide the ad budget by the profit you make per sale before ads. That is how many sales the campaign must produce just to break even. The profit per sale is also your maximum cost per acquisition: pay more than that per sale and you lose money on every order.
FAQ
What is a maximum CPA?
Your maximum cost per acquisition is the profit per sale before ads. Above that, each sale from ads loses money.
How many clicks do I need?
Sales needed ÷ conversion rate. At a 2% conversion rate, 25 sales need about 1,250 clicks.