I currently use Google Analytics, and I want to track the return visitors from my PPC campaigns from Google Adwords & Yahoo Search Marketing. For example, when someone clicks on an ad, do they come back to my site?

The system that measure's a user's lifetime value is only as good as the data that can be captured.

If you have the technology to store cookies on your user's machines, store a cookie that expires after a long period of time (60-90 days) with the original source of the user and a number value that you can increment to count the number of return sessions by that user.

Check for the cookie values on page load, increment the count for the number of visits. Pass these values through to your checkout/thankyou page to find the total revenue that single user has generated.