Wondering why the improvement from 2 to 3 to 5 blades?
Aside from marketing (think megapixels in digicams & dpi in printers), the spacing between the blades get narrower, therefore holding down the surface of the skin flatter which results in less bumps and giving you a smoother nick-free shave.
All Mach 3 blades will fit in any Mac 3 razors and all Fusion blades will fit on any Fusion razors.
Heads expensive? Well, that's how the company makes it's profit.
Question: Do you change the blades according to timing or by "feel"? If timing, how many days/weeks? If "feel", what do you "feel" for?