Depends on the make and how used they are. New sails go approximately:
If someone has a set they used for a handful of competitions, I could see paying as much as 60% of the original price. However for a pair someone else replaced because they were getting old and worn out, I really don't want to spend a lot. Probably not more than $200, and I'll wind up having to replace them over again and I'll be out the $200.
And of course if they're totally shot, then they would not have any value to me. I do have the factory sails.
Still looking for some gently used racing sails. Nationals are this weekend. Did anyone splurge and buy new sails when the old ones still had some life in them? If so, make me an offer and perhaps you can recoup some of investment you just made on the new ones!