The purchasing power of a Credit is pretty clearly meant to approximate a 1977 US Dollar. Adjusting for inflation, that means almost $4 today. But there are two major sources of divergence:
1.
Baumol's cost disease causes the price of services that are not readily automated to increase, relative to things that become automated as technology advances.
2.
Moore's law drives costs of electronics down as technology advances.
The combination of failing to adequately model these economic effects and choosing to ignore them for the sake of simplicity and continuity has caused some prices to diverge, but the current $4 Credit isn't bad.