The AM GP9 is still available in Pennsy:
Looks like model is a DC Scale version from the original run from 1988 or so. AM hasn't used the beige box in decades.
Given it's an older model and is still currently available, I would say it would sell for $100-$125. (I recently paid $95 each for two FP7's from the same era, and those were hard a to find road name.)
Rusty