I don't think anyone's ever going to beat a friend of mine's. He went into a random supermarket once and was just checking stuff around, when he found Super Mario Galaxy 2 there, and bought it for... R$2. (probably around $0.70 in dollars at the time) I still find it hard to believe despite how many times I've heard that story.
Personally, I don't think I've had too many bargains over the years as I'm usually always buying new releases. So I guess $10 for Metroid Prime Trilogy is the best I got.