I read somewhere once a long time ago that one way you could get a baseline for your hourly rate is to first take into account your monthly expenses and then calculate how much you would need to make per hour to match those expenses.
So let's say (for the sake of a easy to work with figure) you need $1000 per month for rent, food, entertainment, personal business expenses, etc...
That's $250 needed to be earned per week, around $35.70 (rounded) per day for a 7 day week, or $50 per day for a 5 day week (meaning a typical 5 business day week of Monday through Friday).
If we go with the business week of $50 a day and break that down hourly for let's say an 8 hour work day that comes out to about $6.25 an hour.
Now, don't get me wrong, I'm not suggesting you charge 6 bucks an hour. Again, this is just a possible way to get a base for your rate. In the example above, you're only doing one job a month. You might end up doing several a month simultaneously or projects that last longer then a month's time. But you get the idea of matching an hourly rate to the amount of money you need for you budget.
On top of that you can factor in project scope (including client budget), your experience, how fast the project needs to be turned over, etc.
For me it really depends on the client, the project, and my time with both. Do I see myself working long hours? Or do I see it being a quick and straight forward job? I've done $15/hr jobs and I've done $50/hr ones. But I'm also still pretty new to the scene.