But there's no limit on fixed costs for games. Presumably production could cost as much as a Hollywood blockbuster and be priced accordingly. Marginal costs aren't always everything.
Games prices have actually come down compared to the bad old days of paying upwards of $90 for Super Nintendo carts, for example.
It is the case that production costs have been shown to be scalable; hundreds of millions go into developing a game like Bioshock: Infinite or the latest Call of Duty, and hundreds more go into marketing and sales. In practice, it is possible for a publisher to recoup that investment and earn more on top, even if it's really difficult.
When looking at the price of games, you have to remember you're normally going to be spending much more time playing that game than you are watching a single ticket's worth of a movie.
A similar argument could be made for physical movie media vs movie tickets. You pay $30 for a dvd, sure; but, you and 5 of your friends can all sit down and watch that DVD in the comfort of your home. 3 years later, you can watch it again, and again. The cost per hour of that DVD can easily, if you're an avid, repeat movie watcher, drop down to the same price per hour that you're paying for a game.
On the other hand, if you're buying a modern FPS for the story only and hate multiplayer, then you're getting about 6 hours of play for $60; and that's about the cost of a movie.