I am not an expert on this subject. I get my information by monitoring what people list their mustangs for on eBay, Autotrader and Craigs list. What I can tell you is that I see classic mustangs listed from $5,000 as restoration projects up to $15,000 as fully restored w. original trim, average price around $10K for a decent ride. Restomods, with Rack-pinion steering, coil over suspension and modernized options can go as high as $25K.
with the 64 1/2 you will be looking at a couple of must-do upgrades, one is changing the single bowl master cylinder and the other possibly changing the 4-lug wheels.
Again, I am not an expert on this subject. What I can tell you is that buying my 67 was one of the best thing I did for myself. Not for its investment value, but for getting into a hobby that I truly enjoy and I get so much satisfaction from doing. Every true car enthusiast will tell you, do not expect to make a profit, nice if you can, but most importantly enjoy!!!!
Good luck, hope this is helpful