A rough guess would be $500-700, assuming the tolex and grill cloth are in good shape and original. Keep in mind, these aren't that desirable for guitar players and worthless for bass players wanting to play at anything more than very moderate volumes.
They didn't sound all that great back then and 50 yrs later, still don't. But it was all there for was for bass unless you got a dual showman cab with JBL D-140's. That said, a collector could possibly buy it to pair with a '65 Bassman head. Funny thing is, there are Ebay buyers who will buy anything that's old and "vintage", so can't hurt to ask for the moon. Who knows? List it for $950 and see how it goes.
I did see a real clean '66 Bassman cab loaded with Jensen C12N speakers that sold on Ebay for $1199, but those speakers are more desirable than the Oxfords in your cab. The blond era cabs are much more valuable and collectible, they're not built any better but they do sound better because they have the Jensens or earlier Oxfords. They also have more "mojo" in them, mainly due to Brian Setzer and those guys in CSN&Y.
I'm picking up an unloaded 2x12 Bassman cab of the same vintage tomorrow for $140 local. The back panel has been cut to make it an open back cab. I'm the first person to make a serious offer on it over the 2 weeks it's been posted.
Edit: Yeah, I'm building a Blonde Bassman so I'm picking this up to go with it.
Having owned/gigged a Bassman bottom for many years, I'm surprised by the "not great cabs" comment above. Not my experience at all- totally reliable and easy to mic up. Use the right speakers (e.g., old Celestion G65s worked for me) and they're great cabs. That said, I wouldn't pay a premium for the Oxford 12T6s, there are many better choices. $500 would be the upper limit, and it would have to be very clean.