First, you have to remember that the price for the Gursky was achieved on the secondary market. Gursky's not actually asking for $4M for his prints. The high price for the Rhein II picture was probably a result of multiple factors, like the fact that most of the other prints in the edition are in the hands of museums, that it's a Gursky picture at the height of a photographic practice that has basically dominated the past 20 years in photography, and lasty - and this is my own opinion - that it resembles minimal art which makes it more relatable for the art buying crowd who would otherwise not spend this kind of money on photography.
I agree with you on Crewdson. I really want to like his work but I just can't. It's just too much of everything. His Dioramas from the 90s have some appeal, though, in a kitschy Twin Peaks intro sequence kind of way.
As for your premise "a fool and his money", let's be realistic for a minute. This whole development is simply a result of a massive concentration of wealth in a very tiny segment of the population. It's not likely that the person who bought the Gursky has only a few million lying around and is investing it in one photograph. It's more likely that he/she (though we all know it's probably a "he") has a billion in their bank account so at this point it doesn't really matter if they spend a few million on something that they want. They have more money than they can spend anyways.