What you spent on advertising to win one customer. Two thousand dollars of ads producing eight customers is a CPA of $250.
CPA is the number to judge a campaign on, because it is the one that can be compared directly against what a customer is worth. A cost per click is meaningless on its own and a cost per lead is only half the story — leads that never close cost you money.
It is also the number that decides how aggressive you can be. If a customer is worth $1,800 in gross profit over the relationship, a $250 CPA leaves a lot of room. If they are worth $300, it does not.
There is a free calculator on this site for exactly this.
A free consult is twenty minutes and there is no pitch deck at the end of it.