Those are beautiful kits.. I think it all depends if you are selling the copyright on how much you want to charge them for it, I think you shouldn't charge anything less the 250 but would charge more if they want exclusive rights to your work (meaning they are resaling them but you can't) The other thing you want to think about is if you sell it to them are you going to get royalties (meaning you get a percent of what they sell of you stuff.)