The market determines the fees. We try to best understand what the market is, where the individuals we work with fit into that marketplace and then we test it out when we're working with someone new by setting a fee. If we can't seem to get any momentum going, it might be a fee issue and we might have set the fee too high.
We tend to go below the market value at the beginning to build some momentum, and then work from there, assuming the feedback is good. It's really the market that determines the fees.