Rasmus, you say you're a freelancer so I'm assuming you've done some freelance contracts in the past. I'd treat it like that. Since it started as a personal project and thus didn't have multiple revisions and external art direction and such, and you don't need to budget for those things, you'll probably wind up offering…
I am completely serious. I don't care about your personal choice, I just want you to understand why you're making it. For rasmus, I like Jay Evans' idea. It's a good idea. Try that. Get them to distribute your model on their developer network with your name credited, and now your work gets seen by every licensee. For…