The best deal I ever got was a boxed Gameboy Color for $1.50. I was in this little thrift store that never has games, run by a group of nice old ladies, and there it was for $3.00. And it just so happened everything in the store was half off this day. They also had a few boxed games I picked up for .50 each. I was stunned.
I'm not sure the exact value of it, but I know an offer was made to me by a retro game store owner for $100. It's missing the manual and inserts, which would increase the value more,on Ebay it was selling for $150.00 with the manual. I don't really like to go by Ebay prices though, cuz buy it now prices don't always reflect what the auctions are selling things at. But even without the box, $1.50 is a total steal.
Needless to say I regularly check in on this place for games now.