Why are NAND Gates cheap?
NAND gates are cheap because there are so many of them lying around from the 1980s.

Seriously though, a NAND gate is about the simplest logic gate.  You can think of it as a multi-input inverter.  Electrically, that's exactly what TTL NAND gates are.  Each input is just another emitter added to the input transistor.  The rest of the circuit is just a inverter.  It's different in CMOS, but a NAND gate is still very simple.

Since the chips require few transistors, they can be small, which allows lots of them per silicon wafer, which makes them cheap.