I don't think you read the relevant bits. The project was put on hold a few years ago for patent legality research. And, they are doing a "soft migration" in which relevant open source applications are being installed on Windows to gear up the user base for the switch.
Just pulling the rug out from under all the users quickly is stupid and will generate nothing but backlash. I read the OSOR page, and it seems they know what they are doing and doing it well. I drive a Mercedes, and I can say that Germans don't half ass things.
Speculatively, I would say the cost is so high because the city most likely dug themselves a hole by developing loads of software that is Windows specific. But, they are doing the right thing here by getting their technology independence. In 10 years from now, their operating costs will be amazingly low since they will ditch millions in MS tax, have a user base acclimatized to Linux, flexible applications, and knowledgeable admins.
This should be an example and business case to other governments and large organizations that they too can save themselves tons of cash by just going through the pain of undoing "easy decisions".