I've cleared polished aluminum, magnesium, bare steel, aluminum, titanium. There are clear "metal etchers" that dont affect the surface. They are completely clear and etch into the metal. You can spray regular urethane or enamel clears over the top of polished metals, but any leftover buffing compounds or grease is going to keep it from adhering correctly. The peices have to be 100% CLEAN. Even then, there is a chance of the urethane or enamel lifting after a few years. Spraying regular clearcoat over the metal etchers is really a better way to go IMO.
Without polishing, just for clearing polished metal I charge $75 a wheel with no prep work (ship them in, ship them back). Frames are $125, misc peices are done on a peice-by-peice. I'd say most shops will be in that general range I'm not setup for polishing, just the paint work. No idea what it costs to polish stuff.