I would peg your fee against the quality of your portfolio.

For a ballpark estimate, try charging an hourly rate pegged at the yearly salary you want to make divided by 1000. My reasoning behind this is the other thread suggesting that contractors need 2x an employee's salary to cover for a lack of benefits balanced against the idea that a standard 40 hours x 52 weeks work year has 2000 hours.

  Example:  $50,000 * 2 / 2000 = $50/hr.