Looking at KEH it seems generally you can get an EX condition M6 for $1,250 or so. You can get a LN- for about $1,600. My experience with KEH is that anything EX+ or better is perfect - YMMV.

In terms of the Zeiss ZM lenses at KEH or on an auction site, LN new ones in perfect condition can be had for 65% or so of the price new.

My experience with KEH is that none of the equipment I have bought there has needed a CLA.