The way I look at it, how much would you like to be paid an hour to make it worth giving up your time to work on someone else's art? For example, when I was starting out I got paid minimum wage at my day job and it was worth it to me to do commission work for $10 an hour. Now my day job pays a lot more and I've become a better artist, which means my own original art is worth more and so a commission would have to pay me more to tear me away from it.
And when I talk about getting paid per hour, that's not always the best way to go. I just use a per hour scale to help me price things out. For example- say you know that a 5X7 pen and ink drawing normally takes you 2 hours and you'd like to get paid $10 an hour. Whenever someone wants one, tell them drawings of that size cost $20. That way the patron isn't worried that you'll try and take your time to squeeze money out of them, you won't be worried that you'll finish too soon and not get the money you need, and no one will be surprised by the bill at the end. You also don't want to loose money just because you are getting better and faster.
After a while you'll get a feel for the market as well. If you price your 5X7s at $20 each and you start to get too many commissions for you to handle that means it is time to raise your prices. If your 5x7s are priced at $100 each and no one has asked you for one in months, then your prices need to come down. Soon you'll find a price that enough people are willing to pay to keep you busy- the highest price the market will bear. This price should slowly go up as you become a better artist and produce a better quality product.
And don't forget what Kissiesama said about shipping and supplies! If it is just a drawing or digital work I don't charge for supplies, but I'm a printmaker and a bookbinder and when I get commissions for items in those mediums I keep a ledger of all the supplies I use, their quantities, and their prices. You can either estimate how much the supplies will cost before the project and add that onto the upfront price, or explain to the patron that you charge for supplies and present them with your ledger in invoice form when payment is due. Whenever I deal with a patron that will require shipping I always warn them that they will be responsible for it and either at a small shipping fee or (often the case with large packages) mail things COD- again, not without warning the patron first.
Whew, okay. I think I'm done. Sorry my response was so long!
It varies from person to person on what they think their art is worth. But, some basic things a lot of people like to keep in mind when creating a commission piece or coming up with a guideline: The average amount of time that goes into a piece, the cost of supplies or shipping (if necessary), minimum wage in comparison to the amount of time for a piece, demand for their art, etc. Of course, this isn't everyone, but this could give you an idea of where to start