A mint pair with nice original grills and stands are easily worth $1,500+. If you can get them for less than that more power to you. The problem with these is that there are so few of them around on the open market that there is no information to use value-wise. In general however, it's all relative to the people involved. If it's fair to you and fair to them then it's a good deal. It doesn't matter what you pay/get for them. I can say that if you tried to buy a new pair you're talking 3 grand easy all day long if someone had something like them.
So far as Adam's comments, regardless of what I paid for them, mine came from Detroit to Charlotte (in 2 trips no less!) so there are other extraneous factors that come into play also.