You can get the rare DOOM Expansion Set for $59.95 canadian which is basically the same price in USD.

www.boardgames.ca/doomtheboardgameexpansionset.aspx

With shipping to the US this will run you about $77. Which is far cheaper than Ebay, Amazon, or Boardgamegeek.

Unless you know of one sitting on a local FLGS shelf, this is the best price i have seen in about 4 months.

And it is legit because that is where I got mine.