I have yet to do any paid commissions, right now I'm still working on getting what I desire out of the digital tools and learning my techniques. Eventually I hope to do some however. (So folks, when you see my stuff feel free to critique with that in mind, I will try to be strong ).
I do however currently own and operate my own business. I can tell you this, as you grow, if your prices are too low you will find yourself swamped with work until you burn out or your quality tanks at which point you will probably quit. More likely however, you will find that you are too busy to take on that next commission and it will become apparent that you need to start charging more just to keep the volume of business under control. It's basically supply and demand, you are the supply and there is a limit to how much you can produce. Of course, you don't want to enter the market too far out of the bubble or you will never get established.
I am sure I will be asking the same questions if and when I start doing commissions. There was a podcast about pricing for artwork on Ninja Mountain Scrolls somewhere but it was weeks ago. Seems artists always have a hard time knowing what to charge.
Before I finished school and went into Corporate America, I did sell a few art pieces I had done (I wasn't doing maps btw) and I had no idea what to charge. The problem for me is I want paid more for doing something I hate, say digging a trench, than for doing something i love. But just because you love doing something doesn't mean it isn't of value and you might be surprised by how much it really is worth.
There are reasons for the "starving artist" tag and it seems not knowing what to charge is one of them.