Metrolinx has released new business cases for the Scarborough subway extension and the Eglinton West LRT. They show both projects have costs that significantly outweigh their benefits, and by Metrolinx's own definition are not economically viable.
Here is the Scarborough subway business case: metrolinx.com/en/regionalpla…
Here's the Eglinton West LRT business case: metrolinx.com/en/regionalpla…
Here are the numbers on the Scarborough subway. It would generate $2.8 billion in economic benefits, and cost up to $6 billion. Its cost benefit ratio would be up to 0.66. Metrolinx's guidelines say projects need a ratio of greater or equal to 1 to be economically viable. 
Despite these negative findings, the Metrolinx report "recommends advancing design and procurement of the Scarborough Subway Extension."
Importantly, Metrolinx has measured the benefits of the Scarborough subway not against a potential LRT, or even the existing SRT, but against a scenario where the SRT is decommissioned and the TTC just runs buses.
