In a word, Yes.
What does he mean by big money? Yes, I have seen a 6cyl at BJ go for $20K. I bet it didn't have a rusty cowl but you never know.
Looks nice in the pic which is never the whole story, if you can stand yellow, yech
, and since all these dogs have some fleas the trick is how many fleas and no looming distemper! A lot will depend on what else you see similar within 200 miles but you won't do much better unless you are ready to accept a mangy mutt.
For $12-15K i would expect a car that runs and drives fairly well, is complete and 20 foot presentable but maybe needs light maintenance and only the most minor rust. If it needs much more I feel you could drop down to the $8-12K range maybe even less for a lesser car and end up doing the exact same work to get it into good shape.
One thing I think that is raising the prices on a generic Mustang are that many of us know that we couldn't replace our cars with any sort of similar "classic" without spending a lot more so we just don't sell even if we can get 4-5 or even 10X what we originally paid. Similarly some people think $15K is a good deal on a crapp box when they see Camaro etc "projects" regularly go for 3X that much. Then there are those that think they deserve to get the money that put into one back.
If you are ready to spend that much I bet you can get a very very nice one by taking your time and willing to travel with cash in your pocket to inspect a few and act when you see it.