I am looking for a module that will track the number of times an ad is clicked and report on purchases made via the click.  Anyone know of a good one?

I don't know of a module that will do this, but you might try checking out Google Analytics.  They have an option for online stores that you can use.  You will have to integrate their code into your online store though.

I know the catalook store has a referral capability that could track if a user came from a certain link and bought a product.

Between the two things I think you could get a very accurate picture of what that banner ad is doing for you.

Stuart

Do you have control over the store at the other site?  Otherwise you have no way of knowing (unless they provide an API, extract, or other means) what the purchase price was.  In general a click-through is only going to tell you that a click through to site "x" happened at time "y" (and by IP address or person "z", depending on how in-depth the click-through software is).

For that matter, you aren't going to know anything about what they did on the "other site" unless you have control over that "other site" and have a way of tracking where they came from (whether linking a "click-through" ID with a session and then tracing their every move; likewise could be done with cookies).

So again, the question is, do you have control over the other sites they'll be clicking out to?  If not, do you have a way of finding that information out (extract, API, etc)? Probably not?