"Some" of the Eglinton corridor can accomodate cheaper construction, not "most" of it. I think that 10 km out of 30 would be an optimistic estimate.
It is true that Eglinton could save something on station costs (only Don Mills, Jane, Kipling, and the airport would need big stations). But this route has its share of other challenges:
- 3 highways: 404, 401, 427
- 4 rivers: East Don, West Don, Black Creek, Humber
- crossing old central part of the city with its numerous utility lines (I suspect that has added to the projected cost of Eglinton LRT tunnel)
- complex wyes for service connections to Spadina and/or Yonge lines
So, I think that $200 million/kilometre for the 10 km of "easy" Richview and East York sections, and $300 million/kilometre for the remaining 20 km, is not an overstatement (perhaps, an understatement). Adding up: $8 B, which is $3.4 B more than the allocated Eglinton LRT funding.