You can still find TONS of cheap games. It's just the focus has shifted. It used to be that cartridges were easy and cheap to get a hold of because people were selling them off to get the new-fangled CD-ROM systems.
Now the people that collect those have bought them up from the people selling them off, and they're holding on to them, making them harder to find than when people were selling them off.
Now, though, it's Xbox, GameCube, PS2, 360, Wii, GBA, and PS1 (though starting dry up) games that are dirt cheap because that's the stuff people are selling off.