Well, maybe off the dealers lots, they may have had premium but, in private party sells, not much. I don't remember a price but, when the Hertz, GT 350 came on the market, they sold pretty cheap. There weren't many, pristine Pony or Muscle cars, most, in those days, were rode pretty hard, at least in my area.
From the 70's to the 80's, there really wasn't a collectors market, to speak of. I had a hell of a time selling my two, K code cars, I couldn't get $600 for my avatar car and the sin I committed was with my '64, K code Sports Coupe.
I couldn't sell it in late '68, when I ordered a '69 SCJ (had to sell for the down payment so. I didn't get it) nor in '71 when I didn't have room for it, after starting racing.
My sin was that I pulled the Engine and trans and had it hauled off, for scrap, it was still pretty nice car. I let the 9" go with it.
The Fairlane and the Mustang, were advertised in the OC news paper (Register) off and on, for about 3 months (twice for the Fairlane). After about 6 mo, I wound up getting a good deal on the Must. A friend from HS wanted it and traded his '55 Nomad for it, I sold the Nomad in less than a month, for $900. And I, for sure, didn't want a GT 350 or 500.