You Yankee fans are really gullible.
It shouldn't cost $800 to replace your entire shower valve body assembly.
It sounds like you have a 3 handle set-up: Hot, cold, and diverter. What exactly is wrong with the diverter? You may just need to replace only the valve stem or even just a washer.
Remove the cold water handle and tighten the packing nut to fix the other problem.