I usually price my adopts cheaper than a custom or a commission, but you could always do auctions which will give you a pretty good idea how much your specific audience would want to pay for them! 

In terms of usage, I think it's important to have a TOS specifically detailing what your designs can and cannot be used for- especially when it comes to things like dating sims, video games, web comics, etc. It's also worth detailing whether or not you're comfortable with someone changing the design, editing them post purchase, or even paying you an extra fee to edit the design for them!

I'm terribad at explaining, but here's my TOS if you're interested in an example!

Getting into adopts can be rlly painful, I think with the scope of your audience you might not have a problem, but it is normal for designs to sit around for a loooong while before the right person comes around who wants it! vov My motto is not to throw away a design, even if maybe you don't feel it's strong enough or if it's not up to your tastes because there is someone out there who will love it as it is!