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