$10 - Monochrome
$25 - Color (may include simple lighting)
$50 - Shading/Lighting
Additional characters add 50% each.
General guides - I just do girls, inflation, enhanced assets (breasts, butt, belly) and other size-related things like muscles (but I'm not as good at those other things).
But otherwise you may try to ask for just about anything, I'll just ignore what I don't want to do. So consider what you really want carefully!
Nothing's really stopping you from submitting multiple ideas though. If I contact you and I choose more than one, you can decline any of the extras if you don't have the money for all of them.
I also just do characters, backgrounds aren't my forte, but I can do simple things like skies or patterns.