Not going to beat around the bush here: the HP 2311gt offers passive 3D for only $260 (at the time of this review). From my experience, that's a pretty good price for a 3D monitor.
Sure, the 3D effect requires that you sit at least 2.5 feet away from the screen, but when it works, it's actually surprisingly convincing.
The question is: is it worth it?
The HP 2311x is pretty much the same monitor, except without 3D features; however, it costs a full $90 less. While $260 may be a good deal for a 3D monitor, you're paying a somewhat fat premium for it.
Still, it's one of the cheapest ways to experience 3D content, this side of a $16 movie ticket. Check the full review for more details.
More reviews can be easily accessed from the monitor hub.