Really depends on the size.
There's industrial-style powdercoating, such as what an amusement park would contract out to have 3,890 feet of railing powdercoated for the railings in the lines you wait in, while others would just paint the railings... there's places that powdercoat whole car frames, and then there's places that will do wheels only up to 16 or 18" max... and yet others that will charge a premium to do 20, 22 and sometimes 24" rims.
Lastly, there's SO much variance in pricing that your best bet is to call around.
I can't get my plenum polished around here for less than $400-500.
If I were in Chicago looking around, I could probably find a place for $100-200. Good luck, and be sure to use a place that has personal recommendations as to how well it holds up.
FYI, many 2-part paints hold up just as good if not better than powdercoating. In the Navy we use a 2-part paint on the decks, it's epoxy style and we call it red deck (because it's red and goes on the deck, making the deck red, go figure) and that stuff is semi-indestructible. Get it on clothes and it will never, ever come off. Get it on your fingers and you may have them stuck together for a lil while. 2 part paints are no joke (the only caliper paint you should use is 2-part style).