Keep in mind that the government raised the price of postage for the duration of World War I by a penny, then moved it back to 2 cents after Harding took office. So, in that case, $3 x 50/2 = $75.00.
The stamp above was issued for use during World War I. A comparable 3 cents stamp was issued for World War II, but from this time forward, there was never another rate decrease. It stayed at 3 cents until 1957, when it was increased to 4 cents.