Did you have an idea for how you would be logging hours spent working on this that would be completely transparent to the person you're working for? I'm just asking because when it comes to $50/hour wage, every few minutes spent that you may have left the computer for a meal or something along those lines can add up (in your favor and at the expense of your "employer").
That's a good question.
I think it's a matter of trust, at first. There isn't a logging software (like oDesk is doing) that will really prove you are working. You can be just moving your mouse around. That being said, I'll make an estimate (in hours) of how much it'll take. It should be reasonable and if it's going to take a lot longer I'll explain that to the client.
I might spend 2 hours doing a 1 hour job. I'll charge only 1 hour. I do the estimate based on production I do when I'm in my average productive state.