US inflation was a factor of 5.7 from 1973 to 2018. Inflation was higher in the 1970s, but even from 1981 to 2018, a period of low inflation, inflation has compounded to a factor of 2.8. If we take that $325 used M4 or $627 new M5 in 1973 and multiply it by 5.7, we get $1850 and $3570 respectively. HIgh-end cameras were really expensive. I'm not exactly sure what the market prices are now, but a used M4 seems to go for around $1000 now, and I'm sure an M5 isn't anywhere near $3500. Cameras, like many collectible items, are lousy investments.
On the other hand, if one had bought the M4 in 1973 to use as a tool for image making, one would have gotten to use it for 45 years.