Like many artists, I've always wondered "how much should I ask for my work?"
There's a lot of things that go into what you should charge, and there's never really a 'right' answer. Hopefully these points I talk about might help you in some way...
Time vs Effort
Whenever the question of pay comes up, I always consider how much work I need to do and I do a few quick calculations in my head. If the amount of work seems too much for the amount of money it is, I will explain that I won't be able to do it for that little. (It's that simple).
If you think the amount of work is just about right, or a little under what you expect, you can still agree to it, but make sure not to spend more time on it than necessary. I know you're an ambitious artist...stop being so ambitious...
I need to make a 10 backgrounds, a main character, animate that character, and make some user interface stuff. That's already a ton of work, and I'm told to do it for around $300. To me that seems very unreasonable, so I renegotiate it to at least $500. Still low in my opinion, but I'm able to reduce the amount of effort I put into it so I'm not overworking myself.
If you're still in school, your rate will definitely be lower, or you might have to spend extra time on the project because of your lack of experience. Conversely, if you're an expert in your field, you're free to charge more because you know what you're doing. Showing confidence in your field is almost as important as your actual skills, because that's how you'll get work.
Non Artist Bosses
Creative people have to deal with non creative people, accept that as a reality. You're going to be questioned about why you need to be paid what you're asking for because people don't understand how much it takes to create art. If you come off as not thinking visuals are very important, you might as well quit being an artist because no one's going to hire you with that attitude.
Visuals are one of the most important things - it's the first thing people see, and if what they see is hideous, that will turn down a significant portion of people.