I had a mirror resilvered (or more precisely, re-aluminumed) for my Philco projection TV. This outfit does a good job, and won't break the bank. You'll see they show prices right on their website:
http://www.spectrum-coatings.com/
The symptom of a bad mirror is poor contrast. Too much light scatters off the rough and dirty spots on the mirrors, putting some light into the dark areas of your picture that doesn't belong there. It will not cause bad focus, so that is a separate problem to be worked out.
Here's a thread on ARF from when I got my mirror recoated:
http://www.antiqueradios.com/forums/...524069#p524069
Later I made some further improvements to the set, that resulted in better pictures than I showed in the thread linked above.