My two cents on the subject:

If a client needs my services as a freelancer, he will necessarily be paying a higher hourly rate than he would if he employed me full time. Note that I am not an illustrator, but the principals involved are pretty much the same.

A client needs someone to configure a computer on his network. He can either:

1) pay me $125 per hour for my services

2) hire a full-time employee for $80,000 per year

3) try to do it himself, screw it up and then pay me $125 per hour to un-screw it.

I am not asking him to pay me $250,000 for a year of full-time work, I am asking him for $125 per hour.

Of course, the stock-house prices are a good place to start, but if the stock house had the illustration the client was looking for, he could just by it from Getty and be done with it. This client is looking for custom work and should either pay for it or hire a full-time illustrator to produce it at a lower hourly cost (leaving the client with the question of what to do with the illustrator the other 1990 hours of the work year). 

Phillip