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) Station upgrades are expensive, if you are operating frequent services many stations need to be rebuilt with level platforms, elevators, etc. etc.
4) Rolling stock is very expensive, especially for a network that large! You will easily need >1B in rolling stock alone probably much more.
Montreal has the unique advantage of being able to see Toronto, the time it takes to fully upgrade the network to something like an S-Bahn is very long, especially if you want to keep running trains at the same time (and if you do it will easily take 15+ years to do the whole network because you cannot get enough workers to do everything at once as Toronto has found). My view is that the REM is a far better model when we have so little track and row ownership. With REM Montreal will get regional electric trains 5 years sooner than Toronto (despite starting 10 years later), the stations will be universally higher quality, services will be much more frequent as well. This means in the first 10-15 years I expect REM to easily outpace GO for ridership. Once the initial REM fills up another line can simply be built, and if the CDPQ makes it rich on the first - they'd probably fund it.