Wasn't that explained in inordinate detail further up this thread, or in one of the other Transit City threads back in 2009 or 2010?
I recall a fair bit on why our tunnel boring was more expensive than others (larger diameter tunnel, more difficult soil condition, inflation), but I did not recall the other ideas discussed as much. Instead of comparing our tunnels to others, why not compare our grade-separated transit with capacity of 10,000/hr (expandable to 20k) to others.
Would Scarborough have accepted an elevated line - I do not recall it even being on the Table. Of course cost must come into it, so the question is, do people want median LRT (plus the burried section) for $6B, or fully underground for $9B (I am adding some to get the line to Sheppard as with the median/surface option), or $XB for an elevated line. If X=$6.5B, maybe elevated is the way to go, if X=$8B, then median LRT is better. If you want people, or councillors, to decide on this, they should be aware of all viable options to make an educated vote - and I think elevated is viable.
I may be able to make the same arguement with cut and cover instead of bored. If the cost of cut-and-cover undergournd is $6.5B, then maybe it should be considered. Would this leave one lane in each direction? Possibly business along the route could be compensated with some of the savings. Since funding is dictating the speed of this project and not engineering, perhaps shorter segments could be built at a time to minimize the disruption - and the contractor may be able to put more effort into finishing each short segment faster.
Maybe if Metorlinx builds this with P3, they could ask for proposals and decide beforehand what the benefit of a grade separated line (or burried) would be and give the tolerable lane closures and durations.