Inflation can have a terrible effect over a long time. The Reserve Bank of Australia (that's the Australian Government banknote issuing authority) maintains that an inflation rate of 3% per annum is perfectly acceptable for a healthy national economy.
If the inflation rate of 3% was applied to that $50 US bill over 60 years, the value of that note in 1950 dollar terms is US $8.49.
$US70, which is the value suggested now for the note in a collectors market, would have a value of US $11.89 back in 1950. It certainly would not be worthwhile hanging onto that note way back in 1950, but to spend it at the time.