I agree that they are rather steeply priced. In mitigation, I can certainly tell the difference between a humble Bic Orange razor and a Gillette Mach 3 so they are doing something right. Plus the new blades seem to last forever before going blunt - a pack of four can last me two months and I shave (my face) every day!
True but buying trial packs often works out cheaper than buying new heads so one could chop and change at will if one shopped around. PS If you do use a Mach 3 razor I have been told that Asda do a replacement head which is identical in all respects except price - I don't shop there so I cannot confirm this though.
Razor blades are chemically sharpened which puts the production cost up. However, the companies love to charge as much as they can get away with... and they usuakky do!