I was going to use these guys --
http://iphone-icon.com/ -- who charge $49 for a 2 day turnaround.
I ended up using
crowdspring for some UI elements and the designer who won also threw in my icon. He's actually done quite a bit of work for me and I've had nothing but positive experiences.
To your original question, I would say no more than $75 and that's if the icon is fairly complex and you want a lot of revisions and refinement.