I can't give hard numbers because no MMO maker has ever released them, but I do know the pieces of the picture.
With an MMO you're paying for:
1. Server Hardware
We're talking the big-guns too. Servers that cost hundreds of thousands of dollars EACH. Now, each realm represents a FARM of servers to run. Each realm probably represents over a million dollars in initial hardware purchases.
2. Networking Infrastructure
I bet there are a lot of really nice, really expensive switches, routers and special load-balancing equipment that keeps the data flowing smoothly. That stuff ain't cheap in the least.
3. Maintenance of above hardware
Computers break, need to be replaced, or added to frequently in situations like this. We saw bad capacity problems at the beginning because they underestimated load and had to significantly expand all the realms to account for the load. That was a pretty penny no doubt. Plus, you have to pay network engineers and server admins to keep it all up and running. In CA, those guys ain't cheap.
4. Ongoing development
Patches, balacning, adding features and content. That takes more than a few people to do. They ain't cheap either.
5. Community Management
You have the web and forum teams that keep the populace happy (or at least quiet) They're a growing team of people. They cost money too.
6. Customer Service
With 4m customers spread over the globe, there has to be a small army of GMs and customer service reps, and they ALL cost money.
7. Bandwidth
Bandwidth is always expensive, and this game uses a LOT.
So in addition to just the hardware and infrastructure costs, you're paying for a LOT of people's salaries, and you're working on paying off the initial investment in the game in the years it was being developed but not making any money.
MMOs cost a LOT to keep going on a daily basis. Major content additions deserve being a separate boxed product.