Honestly, with that many miles, and the condition that you say she's in, that's not that bad. However, pictures would really help us determine if that was too low an offer to accept or not.
If you're not going to do anything with it, who knows, that guy might want to do a frame off resto on her.
From what I've seen, high mileage cars anywhere from 80k to 150k miles, and if the body is in decent shape and all stock never abused, can still fetch anywhere from $8,000 to $4,500. It really depends on body condition, maintenance records and if all stock, or if rode hard and put up wet all her life.
Pics and you will get tons more responses on here.
GNs tend to fetch higher values due to most insurance companies only listing the Ts and T-Types as regular Regals. As well as KBB and NADA listing lower values than the GNs. Not may people know about T-Types and Turbo Ts. If it ain't black, not many really take notice. Especially those who haven't a clue about real 80's Muscle cars.
Patrick