Amazon offers Doom 3: BFG Edition for Windows for $2.97 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's a buck under our mention of a download from last July, the best deal now by $4, and the lowest total price we've seen. It features Doom 3, Doom 3: Resurrection of Evil, The Lost Mission, and the original Doom and Doom II games.
Note: It's currently out of stock, however you can still order it at this price and it will ship when it becomes available.