If you can find 3 other people who want to play Borderlands then this is the cheapest way to buy Borderlands at the minute.
The Four Pack is £80.97 which works out as £20.24 per copy.
From the site:
"Includes four copies of Borderlands - Send the extra copies to your friends and be ready to jump in 4-player co-op when the game releases."
This is a Pre-Order deal as the game isn't released on Steam until 30th Oct.
I'm not sure if the Pre-Order offer will stand once the game is released on Steam.
For more information on the game goto:
official site - http://www.borderlandsthegame.com/
review - http://www.metacritic.com/games/platforms/pc/borderlands