Would building a subway downtown be a lot more expensive than in the suburbs? Over the years, I've noticed that that seems to be the general assumption here, but if tunnel boring machines are used, they should clear most of the additional tunnels and utilities downtown without much additional complications.
I would think it would be the stations that would add the majority of the cost differential. Yes the TBMing costs would be about the same, but building a station box at Bay & Wellington is much more complex than building one at Finch & Keele. That's where the underground infrastructure complexities come into play. Also, unlike at Finch & Keele, the bulk of the station box can't be left open for months on end while construction progresses, so that difference in construction technique could also potentially drive up the cost.
Not to mention that even if a TBM is being used, you still need launch sites. Clarence Square (Spadina & Wellington) is about the only downtown site along the DRL route that could be 'easily' converted into a TBM launch site.