Modded gear on the used market.

Friends, during my search for a used cd player with a volume control, I have come across several players whose owners have spent hundreds or even thousands on modifications. I am curious what the conventional wisdom is regarding the premium to be paid for such upgrades. I realize that a mod is not a mod is not a mod. But is there some market consensus that a unit "appropriately" modded by a reputable outfit is worh x % more than its original sibling?