"I mentioned in another forum that the city should use some of the development charges collected from this project to help fund the Sheppard extension."
Unless they set up special charges, the city is free to use development charges from any project to fund transit projects. In the end, isn't all revenue the same?
"Anyway, I was wondering, I've seen people mention a westward extension of Sheppard to Downsview. How much would such an extenison cost and how useful would it really be?"
I seem to bring the topic up about twice a week
I don't see why it would cost much more than $1.0 billion if started today - the total length would be under 4km. You could get away with just the one stop at Bayview (Wilmington/Faywood would be redundant if the Bathurst platform is placed west of Bathurst and if the Downsview platform is placed east of Downsview, and Senlac would merely turn the Bessarion vs Ellesmere dual into a brawl) and no bus terminals are needed...the Bayview bus can be accessed via a transfer on the street and any other routes like YRT might prefer to connect with the YUS line somewhere. The tracks continue a short distance west of Yonge already, and most of the rest would be a simple cut and cover job. One problem is how to cross the West Don...I don't know if a bridge or a tunnel would be better, or how much one would cost vs the other.
Sheppard must be extended to Downsview for the Spadina line to reach its potential. It might relieve the Bloor line quite a bit and could steal tens of thousands of daily rides from major east/west routes like Steeles, Finch, Wilson/York Mills, etc., that suck up a gazillion buses. A Sheppard line from Downsview to STC would see at least 100,000 daily riders almost immediately, and perhaps 150,000 within 10 years (time enough for people to adjust to rejigged bus routes, transit-friendly commuters to move in, towers to be put up in redeveloped areas, etc.). It's hard to say exactly how many people would switch to the Sheppard line for long trips, but Sheppard/Yonge station was triple-platformed for a reason. It's a no-brainer...it should have been built along with the Yonge to Don Mills segment.