Summary: yes, hosting something like the Olympics brings in a ton of money. But the IOC skims off an *astonishing* percentage of it, in the name of running the games -- none of which goes to the much-ballyhooed improvements to the hosting city. The end result is usually a pile of expensive rusting white elephants that are not only useless, they mostly wind up draining taxpaper money on an ongoing basis, for upkeep.
It's a bad deal, highly likely to do Boston much more harm than good. On the plus side, the organizers claim that they will drop the whole stupidity if they lose the support of the residents. So it's time for those of us who actually pay attention to this stuff to say No loudly and often, and to explain that this isn't just catastrophizing: there are lots of recent examples, and they all suggest that it's basically a scam.
(Kate suggested to me the other day that the *right* answer is to stop the Olympics wandering around the world, and instead build a permanent site in Greece, which is arguably where it belongs in the first place. I suspect the politics don't work, but it makes oodles of sense to me...)