Hits from Overture or FindWhat act just like any other traffic. You can get a mix of good hits and not so good hits. A lot of it has to do with your keywords, how it matches your site, and how well your site converts in general. We've worked through several different configurations with hosting scenerios, where we end up paying from $4 to $60 per signup.
If you're not getting any traffic through, though, unfortunately you will probably need to raise your bid. In a lot of instances, you're not going to get any volume behind you hits unless you're in one of the top 3 positions.
I'm sure working directly with adult site keywords is different than a hosting site, but I would imagine the word "porn" is a pretty broad category. It's a little played out, and not specific. When was the last time you typed "porn" into a search engine. If you're seriously trying to make some money with pay per click SE's, you're going to have to get a lot more keywords, and keyword combinations approved. Terms targetted to your site are going to be easy to approve, and hits that come through will be more qualified surfers. You'll also be able to check through the stats the next day to see what words did you the most good.
Just my
Like anything else, success takes practice and persistence. Everyone has to find what works for them.