Non-retail copies (called OEM), are usually cheaper. Technically, they are supposed to be sold along with a computer, not just the software. The loophole: They can be sold along with any piece of computer HARDWARE. So most places that sell OEM copies throw in something like a junk used NIC or similar, or something even cheaper, just to make it "by the book". Its not in the "spirit" of the license, but it does fit the wording, and it makes it much cheaper. I think a retail copy of 2000 used to cost something like $169... OEM copies usually go for $99 to $119. I don't know what the guy who was selling it for $57 was smoking, but I'd like some of it (notice the page isn't there when you go to the store; I guess that price is NLA).
The more expensive listings were probably for multi-license packs.
BTW, OEM copies come with pretty much everything a retail copy would, just no pretty packaging on the outside. Just shrink-wrapped.