The number one rule in stamps is supply and demand. As Pat Herst and many other dealers will tell you, demand is much more important.

If supply is large and demand is small, the price for an item will be less. This is true of many post-1940 US issues. These stamps were printed by the millions. Lots of collectors already have them in their albums. Common stamps will be common for generations to come.

If the supply is small and the demand is large, the price will be higher. For example, everyone would like to own a #C3a Inverted Jenny. Only 100 exist and fewer than that are in public hands (a few are in museums).

If you need evidence that demand is more important, look at Scott #C3a (Inverted Jenny) and Scott #1610c (the CIA Candleholder invert). Both are inverted centers. The quantity known of each is about the same. However, a #C3a sells for about $250,000 compared to about $20,000 for a #1610c. Demand for the #C3a is much higher. Part of that is the history behind these stamps. But it is proof that demand is everything.

All of this assumes a normal market for the material without extraordinary circumstances. For example, people have tried to corner the market on a particular stamp by buying up all of the available supply. The stamp becomes more scarce as the supply dries up and prices begin to rise. Or in some cases, something unexpected like a natural disaster will wipe out a supply. Those factors aside, demand is everything.