While I'd obviously like to see the whole thing built at once, the eastern portion is most urgent because of the relief that it would provide to the Yonge line and Bloor-Yonge station. The western segment, though, could be built very economically and mostly on the surface. One suggested alignment (the one our group has adopted) had it running west from Front and Spadina along the rail corridor to Dufferin, with stops in the Fort York development area and at the Exhibition. From there it would run north on Dufferin to Queen, and then northwest along the rail corridor to Dundas West station. The only underground section of the whole route is the brief stretch along Dufferin from the Ex to Queen. The western section should be buildable for less than $1 billion.
As I've mentioned, the Wellington-Front line was considered to be the best alternative to the Front/Rail Corridor route. Its main downside was cost, because it was all underground and didn't take advantage of the availble rail corridor. The parking garage was a major, though surmountable, construction issue. Another disadvantage today would be its weaker service to the East Bayfront development area.