Oh, that thing is cool! I'll probably use it to mock up my next site.

Since it's the kind of thing I'd use once every few months or less, and I'd use it for one project at a time, I'd probably pay $10 to $20 one time for 3 or 5 mockups. Then, I use the best one mockup to make a site and quit needing the service for a while. I don't need three free ones since the demo gives me a really good idea of what I'll be getting for my money. Does that kind of pricing scheme sound reasonable?

PS - after I buy that package, email me a few months or a year later to see if I'm ready to mockup another site. Also, refer me to someone who can take the mockups, improve them, and make websites out of them.