well I'm getting mine done through a freind....beers worth more than money ...hehe
but I got quoted around the $25 per rim to powdercoat (place's round Geelong) plus whatever sandblasting costs.
when I had mine done originally a few years back it cost me $60 per rim including sandblasting, I took the rims in to them in the morning and picked them up that same afternoon. Coating places wont do the work for you, there not mechanics and dont have tyre machines, so you'll need to organise taking them off your car, getting the tyres stripped off at your local tyre shop as well as removing the valves an any wheel weights, then take the rims to them as bare rims.
Hope thats of some help. Cheers