I looked at the Metrolinx's "Sheppard-Finch LRT Benefits Case" study. Table 7 (page 35) lists the capital costs of the 5 options considered, in 2008 dollars. Let's compare Option 1 (separate Finch West and Sheppard East lines, no subway extension) to Option 4 (Sheppard subway extended to Downsview, and Finch LRT goes to Downsview instead of Yonge / Finch).
Option 1 is costed at 2,220 million, Option 4 at 3,650 million.
LRT in Option 4 is 2 km shorter (Finch / Dufferin to Sheppard / Alness, rather than to Finch / Yonge), and hence should cost 2 x 70 = 140 million less.
So, the cost of subway (Yonge / Sheppard to Downsview): 3,650 - (2,220 - 140) = 1,570 million in 2008 dollars.