I found this on a blog today, and I thought it was pretty good. It's an hourly rate calculator from something called FreelanceSwitch. While it's intended for craft-type people, it's generic enough for anyone to use:
I put in some WAGs for things like insurance and office rent and got an hourly rate of $110. Then I realized I didn't even include health insurance, which brought it up to $122 per hour. Then I realized that the chart doesn't seem to take taxes into account, which brough it up to about $135 per hour. And that's to make a pretty unimpressive $60,000 or so per year. This is more than I charged per hour in the public sector, but OTOH there are no economies of scale for one person in office space, software, etc. (after all, a full ESRI ArcInfo license is over $10,000).
Also, the blog had a link to a decision graphic called http://shouldiworkforfree.com/. Should you work for free? Probably not, unless it's for your mom.