I picked up this vintage Lego set at a local Lego convention for $35:
The box is open, and not in the best condition. But all the pieces are inside and still sealed in their original packaging!
This set came with some cool pieces, and all of them (even the ones that are loose) are in mint condition!
The reason that this kit was so cheap is that it was designed for younger kids, and collectors want the older kid sets more because they are more popular. The date on this set is 1987, the set number is 337, and the original price was $13.99.