Whatever you charge, don't finish too soon. A lot of rich people seem to get irritated if they think they paid too much money for too little effort.
I went to school with a guy who inherited the family business. The business was construction-related and there was quite a bit of manual labor involved with certain jobs.
One day he hired some guy off the street to work on a specific job. The job was chipping dried concrete out of the mixer trucks. This is a filthy, hot, backbreaking job - inside a mixer wearing a respirator so you can breathe. The deal was $50 a truck under the table, and there were 4 trucks. It took one of the regular employees a day to do a truck, but they hated doing it and would always find an excuse to do something else.
Well, the guy was a dynamo. He finished all four trucks by noon, and came in to ask for his money. At first the boss didn't believe him and went out to inspect the work, which was done just fine. Then the boss told him he wasn't going to pay anyone $200 for a half-day's work, and said he'd pay him $75. He thought that was "fair" since it was over $15 an hour. End of discussion and the boss went to lunch.
Luckily, another guy overheard the whole thing and as soon as the boss left, paid the guy the full $200.