It is more than E85 a year, as this is only the upfront cost, excluding renewal of licenses.
The amount is small on a per-employee basis, however that E6 mln that the city saves can now be used for other purposes. If there's no benefit of using Windows over Ubuntu, this E6 mln (or more, over time) becomes a waste of money. Explain that to your voters, why you'd throw millions of Euros to some foreign company for some unnecessarily expensive product!
And why all or nothing? Because it makes the work of the IT staff a lot easier. Standardise computers, give them all the same hardware and software, and the bulk of the office can do exactly what they have to do. Maybe put in some non-standard (higher end, different OS, whatever) machines in the mix for the people that really need this - this are probably also the people that need the least support, so not much of an issue there.