It's mildly surprising to me how complex the process actually is.
First step: Look at the entertainment that is available in your area, and compare it to what you offer. Be brutally honest here - and get other people's opinions, too. You need to figure out what people who offer what you offer are getting paid in that area, so you know the baseline. The reason you want outside opinions as well is that it's very hard to be objective. Also, don't get defensive if someone says you're not worth what you think you are, take what they say and examine it to decide on your own. Sometimes it takes and outside voice to point out why an act isn't worth what it thinks it is.
Second step: Determine the amount of money that you will be satisfied receiving in exchange for your show. This is the amount you -want- to receive, and the amount that would make you happy. It may or may not be the amount you are able to charge at this time.
Third step: Pick a price that is a good compromise between your goal price, and what is in that area. This may mean you're a bit below what's out there if you're new. You have to develop a reputation.
So! Using that system, I started out charging $300 a few years back. People tipped me, on top of that, so I upped the price. I don't get extra tips now, but people are happy to pay me what I charge.
I am at a point where my hour long show brings in money that makes me happy to do the show as well as deal with travel and schlepping gear back and forth. Now I am just focusing on building more of a reputation and finding more venues.