If I may opine (I gave a talk on this very subject at Transit Con) where I compared the GO RER project to REM.
1) GO RER as a whole will cost something like $40B - acquiring track is very expensive (think about the value CN and CP see in their tracks and corridors - and Montreal given the geography should be even more constrained than Toronto)
2) Grade separation for heavy rail trains is far more expensive than for a metro, and you will want a number to operate high frequencies, if you are fully grade separating at the prices which we typically pay it's easily billions alone.
3) Stati