The $20 billion was just a pure guess to be honest. But, based on the track record of the project, I would (half jokingly) say it is just as accurate as the "official" estimates have been so far. I would love to be proven wrong, I guess we will have to wait and see the results of the RFP.
In terms of what else to spend the money on, there is an endless list of libraries, rec centres, fire halls, road projects (yes, car oriented, but movements per $ may come in as better value). Or, maybe, and I know this sounds crazy, but we could lower taxes? Keep in mind, we are just talking about the upfront capital costs of the line. Has the official estimate of a $40 million a year operating loss for the train been updated/improved, or is that a future cost we can look forward to with the opening of phase 1 still?
I suppose my point is, transit is good, sure, and vital to a city. But not at any cost.