im not sure.. I know when they do them in columbus, the busses get repainted, engine in-frame, interior refresh, everything is disassembled and cleaned (A/C, heaters, wheel bearings, etc.. new steering linkages installed, brakes, fluids ,etc).. my buddy didnt know if they did anything to the transmissions or not.. I think those are rebuilt on an as-needed basis.. air suspensions arent replaced unless there are leaks or rubber cracks in the springs..
the transit authority must consider it worth it as opposed to replacing the bus.. I dont believe they re-furb their hybrids.. I forget why. there was a reason.. probably cost related.
alot depends on whats under the hood.. if its a CAT it will cost more to refurb, esp with a Voith transmission vs an allison transmission.
the city does alot of their own major repairs.. esp now that most all of the fleet is gillig low floors.. they ditched all the RTS's, Neoplans, and the few flyers they had.. my buddy only drives Gilligs.. the new ones are all being bought CNG.. their life-span is still in questrion as all the CNG's in the fleet are less than 10 years old..