Used: $600,000
The stamp was sold in 1975 for $23k, in 2026 that is roughly equivalent to $145k. The $600k realisation would have equated to a compound interest rate of 6.6%. If, at the same time, one had invested in 3 month US treasury bills, over the same period, the average rate of return would have been 4.85%. For 6 month T-bills it would have been 4.48%.