Wow, this is actually a toughie. Let's see... If it's bang for the buck value, I picked up the GOTY copies of Borderlands 1 and 2 on a Steam Sale for $8 total a few summers ago. Otherwise, I think I paid something like 25¢ for a used copy of Star Fox Assault at GameStop once, to replace my old launch-day disc someone ran off with.
I was really baffled honestly.
The seller was a dad, and clearly didn't know the value of these.
I even got a GBA SP (AGS-101) and a fully functional Nintendo Gamecube, including the Pokémon games, for a grand total of $100
While not the cheapest game i have ever bought ( that would be a copy of FF13 i bought for 10p to be used in as a prop in something) the best deal I ever got on some games was a box of PS1 games when a local business whet under. I paid £50 for the box and it had about 25 games in. In the bottom of the box was 2 copies of Suikoden 2 - One opened, one factory sealed Sold the open one almost immediately to a friend for £45 still have the sealed one to this day.
I bought a couple Super Famicom games out of a basket for 30 yen each a few years back. They were useless horse racing and mahjong games. All I wanted were the cartridge shells (to make a home for my SD2SNES).