If you can't do professional quality work, don't charge anything until you are able to. : ) Then when you're able to, charge pro rates and knock off half the price for your friends.
I do a number of odd jobs for people, and I find that if you really can't bring yourself (that is, I can't bring myself) to come up with a dollar figure to start with, just figure the time by an unskilled laborer's wage. That's not to say you or I, or anyone else, is unskilled, but more-so saying "well, I guess I should be making at least this much" - It's at least what you'd be making flipping burgers.
Fretjob takes two hours? 2 * $7.15 (Michigan Minimum Wage) = $14.30. If that method doesn't work for you then man up and grab some cash for yourself!