Just to be fair here...in that comparison........
The original tender for the Broadway extension was signed six years ago in 2020.
According to Stats. Can. the Construction Inflation index would have lifted the numbers by about 90% in that period.
Worth noting then, as the Broadway project is winding down, that the original tender in 2020 was for 1.7B and change.....and the current cost is estimated at over 2.9B . So that project is ~70% over budget.
When calculating even a similar project, you're going to calculate an estimate based on construction inflation forward costs over the life of the project. So you'd use a weighted average over the next 6 years or so. I don't expect Construction Inflation to run anywhere near as hot in the next 6 years as it did the last; but given uncertain global events and spiking energy prices you wouldn't want to be too conservative either.
The tunnel work here is also uniquely complex in that it involves lowering the grade of the existing tunnel and track. On the other hand, I believe this line can zero new fleet costs as the TTC has sufficient spare streetcar fleet to operate the line which is a savings.
At 60% design, which is where we are today, we should have a Class 2 construction estimate. The accuracy range is -10% to +20%. If they've done their planning properly, the 3B should include +20% over the true estimate. Which would mean they are hoping for 2.5B.
None of that is a free pass here, for a fully priced design, just an explanation to put it in context so we understand where it comes from and how to evaluate it.