that link's to a pretty nice car for $12,500. I've been shopping for a 70 for over a year now that's the ballpark for a nice ready to go car. Car's without shakers are a little less, and the big blocks go for a little more.
If it needs floors, how about the frame rails and trunk? No "major" body damage, but its going to need to be painted right? Anything like a fender or door need to be replaced too? I would say an otherwise complete 69 needing floors, engine and tranny would be in the $1-3k range, especially considering you can generally get one complete and ready to go for $10 or 12k.
Unless you got a connection for some deals, I would say plan on at least $1000 for the toploader and associated bellhousing, driveshaft, clutch,etc, $2-3k for the engine if you try to make it look original, maybe a little bit less if you don't. The floors/frame/trunk can be pricey unless you do the work yourself, plus you're still going to need paint and small time stuff like carpet, bumpers, etc and all that chrome stuff and rubber weatherstripping, etc adds up, and that's not to mention if you need upholstry or a dash pad.
I paid 2000 for my 68 fastback with no engine or tranny 8 years ago and it still has its original floors, frame, and most of the trunk. It was a very solid body, and I happened to have a 68 302 block and tranny from another deal sitting in the garage so it worked out great.
Hopefully this guy doesn't think he has a goldmine just because the hulk of a car has Mach 1 on it.