depends on your level of experience. In San Francisco, a good freelance rate for a recent grad is around $50-60/hr. If you’re a more mid-level designer you can probably tack on another 50% on top of that.
Keep in mind, tax rates are higher, you have to pay your own insurance, you can’t bill for time spent on the phone with the client or meetings, you have to pay thousands of dollars for your own software, generating leads, etc. Ultimately, it comes down to what the market will bear (as someone else said), but I would highly recommend starting much higher than you think you should.
A good rule of thumb that I once heard is about double what you’d normally be paid as a full-time employee. So if you want $60,000 a year at a standard entry level ID job (which averages to around $30/hr if you work 50 weeks per year), you should charge around $60/hr as a freelancer if you want a similar pay by the end of the year. The reason for this is that only about half your hours will be billable each week.