Based on the fact that it the technology is not that out-dated for Tri stuff, and it has Dura-Ace shifters, I would put it at a touch higher than your estimate if there are indeed no issues with it, and it is ready to ride. If it needs a tune-up or anything, then maybe your high end estimate is about right.
The fact that it has 1 less gear in the cluster is about the only significant difference between it and a new bike... My gut feel before I looked at the specs was about 1/2 of retail (based solely on age) and I would go with that. About $700. The main down side is no warranty with this used bike.