If you just want to be a small single-person operation running out of your home as a sole proprietorship, with no real 'business' presence, then I'd say $30-$50 an hour.
If you are going to start a 'real' company, do advertising (Yellow Pages is your friend,) and drive a reasonably decent-quality car with the company name on the side, you can get $75-$100 an hour.
But, I run a computer consulting company, and I get less than five Mac jobs a month. (And I employ three full-time technicians.) In Portland, Oregon, where the Mac has a higher-than-average market penetration. If you want to make money, learn to remove Spyware from Windows machines...