the big varible in cost with powder coating is the material you pick. mulit stages cost more than a sigle stage wich is pretty obvious. if it is not standard they have to clean their stuff out to run the new color. also if doing a custom color the more stuff you do at the same time the cheaper stuff gets. basically you get hit with kind of a minimum charge if you have to buy a custom color.
so If I were doing it I would look for some other things to do at the same time. you may be able to ad some items in for free basically.