If you are running a pay-per-click (PPC) advertising campaign, you are (hopefully!) tracking at least your total cost vs. total sales, to gather some type of estimate of your return on advertising spend.
If you're even more on the ball, you are taking into account additional sales costs (COGS, etc.) and from there are calculating a more thorough return on investment.
Now if you are a pro, you are also looking at the lifetime value of a customer acquired through your PPC campaign. to get an even bigger picture of its value to your business.
But how does one even attempt to calculate that kind of metric? Here's an interesting article that helps with that very question.