You forget about all the money that developers get from sale profits. The games budget isn't the only income source for the developers. Bungie probably has millions and is still making money off of Halo. The 500 mill was more than likely mostly used towards R&D for the game, not paying the employees.
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When you measure the performance of a development product or project you almost always measure it against the expected costs. If there are significant changes to your assumptions, i.e. developer salaries skyrocket to 150,000 a year, you may update your budget. You don't get to offset your revenues to justify spending more money. You are thinking of a cash projection. Two different beasts.