cost will vary wherever you go... could be anywhere from $30-$100 a wheel... Like Cruzin said, check around in your area.. or you could ship to a lot of places if you'd want
did a quick google and this place actually lists some appx prices:
http://www.bmrfabrication.com/pc.htm
I'm thinking about powdercoating the inners on my BBSs too, they're lookin pretty ratty.....
let us know how it turns out