This is the type of question that will vary enormously from region to region....indeed, even from city to city.

^^^ as rhizo said, this is totally region dependent. I went full time after getting laid off in '08 so I couldn't even really go by what was standard for the area because those folks weren't working either. So, I used a freelance rate calculator (see the link below) to set my floor. Honestly, I should probably go back and redo it now that I've been in business for four years and have a better idea of my billable/non-billable split (hint: it's never as good as you hope).

Doing it this way, you may be above or below the market rate but at least you know you're covering your costs. If that prices you too high you know you need to reconsider some things, because your biz better cover costs. If you're low, you can start bumping your rates up. I've bumped my rates up by $30/hr since I started and realistically - given how efficient I've become - I need to bump again.

Here is a link that might be useful: Freelance Rate Calculator