That, in a nutshell, is the theory of rational expectations, which proposes that a market of rational actors will value things correctly. The price, therefore, represents the proper value for everything at any given time.
However, the overwhelming evidence shows that we have lots of reasons for believing things, many of them decidedly irrational. We were wired by evolution to survive, not to make economic transactions. The result is that price and value often become uncoupled by fashion and zeitgeist, leaving much more than $20 on the ground.
Personally, I believe that Amazon is a great company and that Jeff Bezos is one of the visionary geniuses of the digital age. They’ve prospered through good times and bad and innovated like few others have. From e-commerce to e-readers, personal publishing to cloud computing, this is a company that can seemingly do it all.