I don't know a ton on the subject but i do know custom patterns *easily* cost more than the plushie...as others said A. you need to do tests but B. you're giving them unlimited ability to reproduce the design. Even if you say they can't sell it, that's hard to enforce.

One place I'd ask for help is the forums on Etsy, there are a number of people there that make all kinds of commissioned 3D items and you're likely to find an answer.

It might be best to look at other plushie-maker's policies to get an idea of standard business practices.