[a-priori]

The important part of that is that the OP says "This is the minimum rate you should charge per day."

You should actually charge as much as you can get away with, as much as people are willing to pay you.

[scottshea]

Agreed. A friend of mine keeps raises his bidding price as he gets overloaded. If they pay the amount then great; if not it is better than saying "no" to potential business.