When I started taking commissions I asked all the questions that people are asking here and the answer I got from other people who'd been doing commissions for a while was start at $10/hour and then as you get more business, better at what you do, think about raising your rates with new clients.
I provided paint jobs of all quality from tabletop to display pieces. This is also important to take into consideration when taking commissions. Your TT and Display prices shouldn't be the same. More time goes into display than TT.
I also wouldn't advertise starting prices. Advertise your hourly rate. If someone sees the starting price and emails you they may be expecting the price they see advertised, not that it goes up from there.
Ghool--you could probably make a little more than you are now. Your a good painter and I'm sure your stuff has gotten better in the last year.
Also remember that if you are a competitive painter or freelance for miniature companies you can also revise your rates after a while. Advertise your credentials on your website/blog as well. Potential clients want to see examples of work in a nicely presented portfolio, they want to see your reasons as to why they should go with you instead of someone else. A lot of people actually go with the idea that you get what you pay for when commissioning a painter. The more you hone your skill and consistently crank out good work, the more you can charge.
I just want to remind people to maintain a civil conversation. Matt and I have actually been really impressed with the discourse on this topic and encourage discussion.