Cheapest individual purchase, or cheapest price when divided by the number of games in the package? Because for individual purchases, the absolute cheapest would be Civilization I, which I found in a record shop for $1, as well as Civ II, which I picked up in a dollar store a year later. Cheapest for the package would probably be Civ I again, because there was another game on the disc, making it $0.50 for each game. The first three Heroes of Might and Magic games are a close runner up, because I got them at a thrift shop for $2 total, or roughly $0.66 a piece. Cheapest recent game, not counting the free copy of Portal, would be Oblivion, which I got for $2 at the same thrift shop I picked up the HOMM games.
I love buying used.
Edit: Just remembered, I got Ultrabots for $0.14 at EB Games when they were getting rid of all their old/used PC games. That is almost certainly my absolute cheapest. In my defense for getting such a crappy game, it had been sitting on the shelf for years, and it always looked interesting to me, but not interesting enough to actually buy what was obviously a shovelware clone of Mechwarrior II. If I had known what was really going on with the sale, which was advertised as, I believe, an after Thanksgiving sale (it was some holiday), I would have grabbed Half Life and Rainbow Six at the very least. There hasn't been a major store carrying used PC games since =(