Depends on the project; if it's just a one-time image, I'd probably charge about 20$, especially if it was an itty bitty one.  However, pixel art can get really complex and take hours upon hours to complete; depending on exactly what the client is asking for, and how long it would take, the price would definitely vary, and of course, it depends on your skill.  If you're not an established pixel artist with several examples to show your ability, you might not want to charge quite so much.  I've only dabbled with pixel art myself so I have little clue how much work it takes to produce good results, but from what I've attempted, it's nothing trivial.  Anyway, a good rule of thumb is, if it's going to take a long time to finish, and you're competent in your ability, charge accordingly.  If it's something you'll finish in an hour, charge accordingly.  Just don't sell yourself short and charge 10$ for work that'll take days to complete, you know?  It really doesn't attract clients like you'd think it would

Anyhow, here's my entry for this contest: