A local Audi shop (not official dealer) suggested I replace the timing belt. I trust the guy, but his recommendation is to do a significant amount of work - he said it's about 50 parts (replace all tensioners, serp belt, etc). His quote is $2200. The Audi dealer only wants $1400 but I suspect they're doing considerably less work. This alone could run you between $1,100-$3,100. The cost of parts will depend on your engine’s condition. The rebuild could be as simple as replacing the engine bearings and seals, or it could be as in-depth as replacing the camshaft, crankshaft, and piston. With this in mind, budget between $2,000-$4,000 for your engine rebuild. S6, A6 4.2 and the A8 are the same 4.2 block with the timing belt in the front. The newer gen 4.2 blocks use a timing chain in the rear of the block. Thus the need to remove motor to change. I'm thing that the interval for timing chain changes is well over 100k? ExX80n5.