CDL.TO
Moderator
I'm not sure it would save much money. There's no longer any availability of ROW in the railway corridor, so you'd either have to tunnel beneath it, or beneath Front - which I doubt would be any cheaper.
Who says there's no ROW availability? It's extremely wide from Union to the Don. Look here for a pic. There's room for 4 more tracks or so. West of Spadina if there's land for the Front Street Extension, there's land for 2 subway tracks.