Mid-'70's? Shoot, I was getting cheap, FREE cars up 'til about '92. Granted, with exception of one or two, all were just regular cars, not 'muscle cars', but still.......
What worked in my favor at the time was I lived in the tri-state area of MA, CT, NY. MA and CT had strict rules about ANY rust on frames (even true surface rust) and CT had the added 'bonus' that an old car still had to pass emissions. NY was like 'meh, as long as it moves under its own power, you're good!'
Back then it wasn't a thing for a junkyard to give you cash for your car (or it was just beginning). People would end up with an 'old car' that ran fine, but wouldn't pass inspection anymore and they didn't want to pay to haul it away, so they'd just leave it sit.
Also, back then, you could approach someone about their car without that person threatening bodily harm (as long as you weren't a prick about it).
I'd cruise around MA, CT, find something sitting and approach the owner that I was interested in buying it, IF it ran (I didn't own a trailer). By the time I came along, these people usually had the town after them for an abandoned car. A majority of the cars I got were under $100 and some even free. I'd get it, register it in NY, and either flip it, or just thrash it 'til it was REALLY dead.
But, back then, as now, one person complained about the amount of different cars I had coming and going out of the garage I was renting, and the cops read me the riot act saying I was running an 'illegal shop'.
I knew I was moving far away in less than a year, so I just got rid of everything.
Notable highlights:
'78 Creastwood (Dodge Monaco) Wagon. This car had EVERY option on it (I think), fully loaded and set up with a Reece hitch that could pull a house. The only rust was the spare tire well. I REALLY wanted to keep this one, but a patch of black ice and 40 ft of guardrail had other plans for it. Only thing that didn't work was the AC. Paid $200 for it. I've seen similar cars recently in worse condition pull $7K or better.
'72 Coronet Wagon. This was a fun one as it was never meant to be anything but a beater. As loaded as the '78 was, this one was a stripper. It did have a 318 though. Put it through all kinds of hell for almost 2 yrs, and it kept coming back for more. Rust and a very bent body from jumping it finally did it in. Cost? FREE
(I wish I still had these for parts for my current wagon)
The one non-mopar was a '77 Impala 2dr, the one with the funky wrap around rear window. Less than 50K miles on it. Rust? RUST! I could literally rip chunks of frame off with my bare hands. Knew someone that wanted the engine and trans, so I registered it so I could drive it some. Ever have an ENTIRE rear axle assembly try to leave a car? I did with this car. Paid $100, sold the engine and trans for $250 with the stipulation he take the whole car.
There's more.......but I've rambled enough. Neat topic, I miss those days.