Too bad it didn't have the Pre-rola greenbacks in it. I also have heard the vented GT75 sound good. With todays market the speaker can be worht more than the cabinet but if the cab is in good shape I concur with the 1,000 to 1,500 range everyone has suggested.
Cabinets with super desirable speakers like blackbacks/pre-rolas are worth what someone is willing to pay for them especially original cones.
Does it have plastic cab corners on it? That would help pinpoint what year it is. Somewhere in early to mid 70's they started installing the plastic cab corners, before that there were none, I don't remember what the years were. I'm sure someone on here does......