At Pearson? My gosh ... the station alone would be half-a-billion minimum I'd think. You'd have to alter the approach, to allow for a longer station. And also move the Airport Link train, as it would foul with extended platform (the original Link train was designed for day 1 to fit with the current platform). On the bright side, the back end of the platform would be so far away from T1, that it would actually be closer to T3 - so perhaps a walkway to there could be added.
I wouldn't be surprised if a $billion is spent before it's over.