Well, that will depend on the exact vintage it is. If it is an early gun from '64 or '65 and has the correct case with the velour interior, it could/would be worth quite a bit more than one from say 1981.
We simply need more info to give a decent estimate. If it is in a presentation case and not just the box, I will take a shot here though, and say that if it is a '70's or early '80's gun, it should be worth right around $700- $800 as you described it.
