Sims 2, when it was newly released, for $20, because it was mislabeled as an expansion pack at the local Wal-Mart.
Also, Metroid Zero Mission and Metroid Fusion for $15 together was pretty sweet, but they were second-hand and without cases, so I'm sure anyone could find them for that price or better. However, I got the first two Ace Attorney games for $25 after they were already out of print, complete with cases and booklets (although my copy of the second game was Japanese, but the Japanese versions of the first three games have the English translation on them).