They want $250 to put in a new HU and a CD changer?!?! Are you serious??? What in the world do they have to fabricate???
A good installation kit, which would include all necessary wiring, brackets, dash adapters, detailed instructions, etc... would cost $50 tops from Crutchfield. Then just mount the changer to the underside of the rear deck in the trunk, run a couple of wires, and you're all done.