How well were they terminated?
Was it done poorly?
So, if they were re-terminated professionally and competently, then I can't see why it would degrade the sound. And if it doesn't degrade the sound, why would it affect the cost to any great degree?
In my opinion, assuming the re-terminations were done professionally and competently, then there should be very little discount, especially if you are buying them used, which means the cost is already discounted 40-60% from retail. Maybe a further 10% or so at most, would be my two cents worth.
(Now, if you were buying them new, then I would agree with you, and say perhaps a 25% discount would be in order.)
My thoughts on the matter anyway.