It's a coupe, not a fastback, so unless there's something particularly rare or unusual about that car, it's not going to be worth a ton. I'd figure somewhere in the $10-15k range if everything is in good shape. If it's immaculate with all correct parts / numbers matching, perhaps $15-25k tops. The fact that the "A" is missing from MUSTANG on the trunk lid, even though it's a very minor thing, would lead me to believe that it isn't meticulously cared for, so I'd be prepared to look for some trouble areas or things that weren't quite done right.
They're a fun platform and very easy to work on. CHECK FOR RUST. Even if it's completely restored and garage kept, get underneath and look at the floor pans and torque boxes. Lift up the liner in the trunk and make sure there's no rust under there. Ask if you can hose it down with a garden hose (especially the cowl area: the vents just in front of the windshield) and look for leaks on the inside carpet right where your feet would be. Cowls are a PITA to replace to begin with, and virtually impossible to do without ruining the paint job, so if it leaks, walk away (or be prepared for a big bill in the near future). Even if you don't live in an area that typically has problems with rust, keep in mind that these vehicles tend to trap twigs and other junk with water and eventually cause issues that even the owner may not be aware of.
Other than that, it's a fun platform. They're relatively light weight (2700-2800 lbs) and have a big enough engine bay to make some serious power if you're so inclined. They won't handle like a modern car, they aren't safe in crashes, but they sure will turn some heads.