The odd thing about tunnelling - from my chats with peeps here in the UK about tunnelling boring machines (TBM) on High Speed 2 and Crossrail 2 - the expensive bit is the machines themselves and the launch of said TBMs. Extending a tunnel length gets cheaper as you go on, as the sunk cost (the TBM and the launch shaft) is already built - the additional costs are wage costs, electricity and the concrete tunnel segments! Surely it would have been cheaper to have tunnelling end to end than having 4 separate tunnel portals...
(noting that you do need different TBMs for different ground conditions, or if you want to "dig from both ends" to speed the construction of the tunnel up)
The cost of the stations is also interesting - the Relief Line plans at places like East Harbour show full length, full height lower concourse/mezzanine levels - as is common in North America. From plans we've seen of Ontario Line stations so far (Pape) - this seems to be similar.
Other projects (Crossrail, Sydney Metro) are have mined stations at depth with no mezzanine levels, reducing construction costs, complexity while still having emergency egress - or you build the station inside a large tunnel bore like in Barcelona (or soon, San Diego).
Alon Levy criticises the construction of said mezzanines in this excellent piece on
"why American construction costs are so high" - worth a read.