Well, as I said, I'm a hobbyist.  I've never taken any kind of graphic design classes or anything but the content and style of my work is what people are interested in, so I dunno if I've got any right to charge high prices lol.  Though there is a demand in it.

That was kinda my reasoning too - if it's an item I don't like I wouldn't use it again so I'd be like "Damn, I'm stuck with this".  But the person I'm working with seems like he's interested in working with me for a whole kind of series, so I'll likely be using it again!

I hadn't thought of the contract part, but I did request half of the total commission before starting after my last experience.  What if they don't like the way you've done a render?  Do you charge them to correct it or take it on yourself?