The original game was yanked from Steam last month for the sake of the recently released BFG Edition, but it's now possible to buy Doom 3 as a standalone bundle again for $13. Doom 3: BFG Edition, which includes Doom and Doom 2, is still available for $30. However, Doom and Doom 2 are only $5 apiece separately, so we have no idea how this math is working out for the BFG bundle.
Perhaps the premium is for The Lost Missions, or the ability to wield the flashlight and gun simultaneously? Anyway, wait a month and all of this will be $3 during some Steam holiday sale.