You used to be able to get Doom 3 for cheap on Steam, on account of the game being pretty damn old. But ever since the release of its updated sibling, Doom 3 BFG edition, the original is nowhere to be found. You can't buy it.
Well, you can, technically, but only as part of the mega-expensive id Software bundle which includes every game the publisher has ever released. So, no thanks.
The removal has rightly been seen by many as a low move, since there are a number of fantastic mods available for the old game that won't work on the new one. Not to mention the fact the old game was cheaper, and did a lot of things the new update adds—like attach a flashlight to your gun—for free.
Responding to a query from Rock Paper Shotgun, id & Bethesda's PR rep says "DOOM 3 and Resurrection of Evil are now only available through the BFG Edition, which also includes the Lost Mission. If you already own the original versions of DOOM 3 and Resurrection of Evil, you will still be able to access them from within your Steam library."
"We do realize that there are fans that would like to purchase the original version of the game, and we're looking into options for them. When there's new info to share, we'll let everyone know."
Wrong answer. Or, to be more precise, wrong non-answer. The right answer would have been either "oh, whoops, mistake, let's fix that", or "hey, you got us, nice work, ok we're putting it back".
Mod-ern Warfare: Non-BFG Doom 3 Yanked From Steam [Rock Paper Shotgun]