Instead of asking for a fixed price for each comission, I do it kind of differently (so you don't have to pay a lot for a simple face shot), I ask for 10$ for each hour of work.
You give me the subject, the character and a reference image, and I'll tell you how long It will take, then afterwards I give you a sketch, if you like it, pay half of the price I'm asking and after I finish it, you pay the other half.
This drawing, for example, took me about 8 hours, so it would be 80$, 40 upfront and 40 when I finished it, though this was my first real comission so I wasn't really experienced at the field (and the comissioner allowed me to share this piece).
You might be thinking "Can't you lie to me about the time you're gonna take to make it?" You would be right at the fact I could lie, but you would be wrong thinking that I'm a greedy asshole, if I finish it earlier then expected, you'll pay less, and if I take longer then expected, you won't pay more.