Showing your game at E3 is expensive as hell, this article breaks it down
Showing your game at E3 is an expensive proposition, and you need to pay for everything from the space to the design of the booth itself. This article is a good look at what it costs for a small booth with an interesting design, and keep in mind all the work that went into keeping costs down. When you see those mammoth booths with the monstrous video and audio setups, you're looking at millions of dollars.
It's hard to think about the return on investment for something like that. Would Activision sell fewer copies of Call of Duty without a booth that costs these monstrous prices? Part of it is just keeping up appearances; you don't want to be showing celebrities your game or selling your ideas to retailers from a booth that looks thrown together. Perception is often reality, especially at E3, and having a nice, well-designed booth is a big part of that.
Still, when you break down how much companies pay for showing their games at E3, it gets pretty intense pretty quickly.