The physical architecture you are going to need to support so many users is a big part of the problem. Load balancers, login servers, gameplay servers.... those costs are high and they're not strongly related to whether you license the core tech or roll your own (optimization questions aside).
The architecture of your own program to run in such an environment is a big issue also, of course.
frob, that's a really interesting approach to estimating costs. I never considered going by the credits. There's some error built into it since on bigger titles you will have many people in the credits who did not nearly contribute for the whole duration of the project (testers, outsourced work, people who were hired part way through or who left), but this is counterbalanced by software projects always being over schedule and over budget.