You are correct, the Montreal Olympics was a financial disaster for that city. There are some sucess stories as well. I live in a host city, Calgary, Alberta, Canada which held the 1988 winter Olympics. Our Olympics has been considered the most successful Olympics, artistically and financially.
Many of the facilities built for the games are still used today. They act as training grounds for many athletes around the world. As a nation with a relatively small population, we went from winning only a handful of medals in Calgary to settting the modern record of most Golds by a country and second in medal count behind the US.
The legacy of our games has lived on, something we Calgarians take pride in... I hope London feels the same way, years after the games of 2012 are over.