Average prices I've seen for commissions on FA: $5 for line art, $10 for flat colored pics, $15-$20 for full colored pics (including shading), $20-$30 for reference sheets (i.e. pictures describing characters from their colors, anthro/feral forms, and details such as clothing, accessories, etc.). Other art can vary drastically, and almost nothing's done for free. I've been looking for people to do requests for a while, but still nothing. I'm willing to do trades, but I honestly have no idea if anyone would accept full-length one-shots... :/
Also, for a comic, that's pretty good. I'm not surprised at all, though, since people will pay good money for that sort of stuff. (Oh, did I mention that the person who asked me to write for him said he may help me out getting commission trades if I'm alright doing other works for him too? Nothing adult, of course. He's actually very happy with cutesy kid-friendly stuff, and keeps praising me on that. I actually just sent him a copy of the first draft and he said he loves it to death, basically.)