How do you value your time? Remember that you will loose a large portion of whatever you make to taxes. Your rate should be what you feel will render an acceptable profit. Any flat rate you charge should be based off of the amount of hours it will take you. I generally give quotes per project based off of how long I expect it to take me, if I don't know I will give a loose estimate and invoice them for the amount it took me in the end (although I require 40%-50% of the estimate upfront).