How much is invested on a video game's cover art?

I was just wondering, how much money is spend on a video game's cover art (the art that is on the front of a video game box)? Or how much of the budged is usually spend on the cover art?

I'm asking this because I really really like cover art of Call of Duty: World at War, and I was wondering how much money was spend on creating that picture. How much is invested on a video game's cover art?
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