Is there evidence that governments are saving that much more with open source? You'd still have to pay developers...and some cities are going to freeload off others to avoid overhead costs.
To add one other angle: what is cheaper, paying 5 million to a company in Redmond yp provide licenses for opersting systems and officd software, or 10 million to a number of local companies to set up and adapt a slightly less refined open source tool for the local needs? In one scenario the money leaves the local economy, in other the city might spend more money but the money stays local and is used to buy houses or sandwiches, pay handymen or bus tickets or taxes. Where is the long term cost/benefit/ratio higher?