The best thing to do is usually set what you think you're worth, offer what you think you can manage(chibis, portraits, full-bodies, certain mediums, etc), and adjust prices accordingly. If you get too overflowed, consider raising prices a bit or making slots. If you get hardly anyone, try lowering the price a bit.
One thing I suggest doing is having a limited number of changes for free. I've seen some artists who worked and reworked for picky or shady commissioners who sometimes get several sketches of their character for free because they kept wanting it redone. The general consensus is 3-5 major changes, after that $5 more.