The original document has the hard facts: Until 2011 they spent 11.7 Mio Euros on the migration and another 2.08 Mio Euros on workflow optimizations that they tackled at the same time. The calculated cost for a migration of the Windows NT environment to a newer windows version including support and the same workflow optimizations are at 15.52 Mio euros. That's a saving of 1.74 Mio Euros or roughly 11.2%. This is taking into account that most migration costs now have been paid, and savings will subsequently will rise. They also have anecdotical evidence that error rates and ongoing support costs have dropped. It's certainly anything but a canonical example for a botched project.
That's a saving of 11.2% comparing an actual cost to an estimated cost; if the estimated cost blew out (which seems to happen a lot in migrations) then the savings could be even bigger.
Yes. And 6 years (2006 the migration started) is far from "nearly a decade" if I can trust my math. The Munich migration has been attacked, downplayed, doomed for as long as it is running. The article here gives some hard data based on a reply to a request from the opposing political party in Munich that hoped to get some ammo to attack the migration. Instead they got well researched data that shows the opposite - so far they are absolutely in budget, saved 30-60% of costs (as planned), reduced support costs (as hoped) and created a side project called WollMux that tremendously helps in migrating the VB cruft and macro ridden templates that accumulated over the past 10-15 years into a reusable library of centralized functions.
They "deduplicated" a lot of redundant forms (one little example: The act of asking for PTO was implemented in 45(!) different forms depending where you work in Munich - now it is one single unified service for all) and by centralising stuff they can now do useful statistics that simply didn't exist before.
The focus on Open Standards starts saving a lot of time and money in the archives already.
(Disclaimer: I work for Red Hat, have been involved privately in the migration a bit since 2005)
The Munich migration is pretty much the canonical example of how to botch a Linux migration.