I've seen a couple of copies of the 4 track 7.5 ips (Ampex?) on eBay in the last couple of years. I have to admit that I don't check the tape listings all that often but IIRC, the last one went for something in the $100+ range. Most well known jazz releases seem to go for $80+ so I'd say it's rare but not super rare.
If you got it from a thrift store, I'd say that it's one fantastic find considering how much they usually go for in those places.
If it's in good shape and sounds good, I'd hang onto it if you're a jazz fan. If not, put it on eBay and you can't help but get a tidy profit. You could always get a couple of guys in a bidding war and really make a killing!