I'm having it done to some wheels next week actually. The wheel shop in my area quoted me $50/wheel, some other shop said $40/wheel. When I was searching for it, people on other forums were saying it'd be in the $15 - $25 range per wheel, but I can't find anyone who will do it for that cheap.
Alternately, you can do it yourself with a dremel. You'll lose the ability to center the wheels via the hub and you may need to get the wheels rebalanced, but it's not a big deal. I'm going to spend the money because I don't want to screw it up and I don't want to spend several hours grinding away aluminum.