That really depends on the game and its requirements. The funny things is that the games like CoD with very little dynamic content are really only bandwidth intensive – so there is a HIGH repetitive monthly cost due to your connectivity. For a game like Minecraft, which has a LOT of dynamic content, memory and storage take a lot of space – resulting in a high initial cost, or high monthly cost if renting the equipment. Modern hosting leverages virtualization technologies very heavily, allowing hosting houses to find the sweet-spot at capital vs operational costs. For a big-ticket game like CoD or BF, the total can be anywhere from $ 5ish=$ 100s per month cost, depending on how much they are leveraging it as an advertising platform. For more niche games, or games who have people running them as experimental or for the experience, you\’ll look closer to the $ 15-$ 60 per month range, unless they are renting a dedicated server, in which case the cheapest places are in the $ 90 per month range.
Really there are t many variables to count, but it boils down to hardware vs scalability vs bandwidth.
EDIT – Also games which are simply massive, such as WoW will utilize clustering, and while Blizz has WoW running rather efficiently, each realm costs them thousands per month.
EDIT2 – This doesn\’t factor in the labor part of operational cost, as that is usually nominal with the administration etc typically being a responsibility of a customer service or engineering branch. It is only MMOs like WoW or EVE which have dedicated operation staff, which is another sum in the thousands per month.