Just doing the math here on my previous post, and Scarborough is 23% of Toronto's population in 2006. Since property tax revenues by borough aren't readily available (or at least I couldn't find them, if anyone can I can refine my estimate), it can be assumed that Scarborough accounts for 23% of the City's property tax revenue, or somewhere in that range.
This means that Scarborough contributes $836,447,482 of the $3,630,021,745 in property taxes in the 2012 budget (
http://www.toronto.ca/legdocs/mmis/2012/bu/bgrd/backgroundfile-43962.pdf).
It can reasonably be assumed that Scarborough would need an extra $8 billion in tax revenue over the next decade to pay for the subways ($2 billion for Eglinton, $5.5 billion for Sheppard, $0.5 billion for the B-D extension to STC). This means an extra $800 million a year, which is almost the same as what Scarborough pays in property taxes every year.
If you try a longer timeframe, say 20 years, it still requires $400,000,000 per year to pay for it.
If you remove Sheppard, and just concentrate on Eglinton and B-D, it would be $250,000,000 per year, or an increase of 29.89% over the current tax rate. I don't have the data for number of tax contributors by type, so I can't calculate exactly what that would mean in terms of a property tax increase per household.
So needless to say, if Scarborough wants to pay for subways, it wouldn't be cheap. But if it's something that they collectively vote for, who can say no?