Maybe not same model but actually same plane? If it was new in the 70s it is used and much cheaper now. If it was new in the 50s, the used price in the 70s might have been lower than it is today though. That's caused by the minimum price the market defines for anything that's airworthy.

Maybe that's an investment stragtegy, buying used planes when they reach their minimum price, then keeping them, flying minimal hours to keep the engine alive, to sell 50 years later. But any potential gain will be neglected by hangar and maintenance cost.