I normally reckon that a complete non running stored for many years Brit bike over 250cc in the North East (as yours seems to be from your description) is worth $1200-1500.
A photograph would be a help.
You have to reckon that after 28 years someone intent on putting it back on the road would certainly need new tires/tubes, chain, maybe the engine/gearbox is seized etc, all fluids changed etc etc ---in addition the rims may well be pretty rusty, the other chrome in questionable condition etc etc.
If you are interested in discussing this further then feel free to e mail me at
richardabeard at gmail dot com
Richard Beard aka Tridentman