The arctic mk. IV spartans in the 2010 Arctic Rockethog set.
Pretty sure those guys were exclusive to that set.
Note: The set itself goes for about 100 euros on amazon...
According to ebay, right now my Atriox would be worth $333 if still new. So, I guess that. But, if you discount that as a ridiculous listing, then it looks like the Halo 3 Arbiter, Elite Honor Guards, and Mark IV Master Chief from the Heroes line are my most valuable.