50% - 90% of new prices, depending on age, functional and cosmetic condition, seller's motivation, etc.
Sorry, I missed your follow-up post. If it's a true US Stereo Set, that's a mighty fine used price.
New:
CMC 64 ST
Two each - CMC 6 amplifiers, matched MK 4 cardioid capsules, A 20 (or A 20 S) elastic suspensions, and B 5 popscreens
$3625.00