My plan shortens the Scarb Subway by 1.7km, thereby saving ~$1B. And although I think an elevated Eglinton East is a great idea, it (combined with an SLRT) would create quite a circuitous Crosstown. From Don Mills to STC it would be 11.3km, whereas using the rail corridor and Ellesmere alignment I've proposed it would come in at 9km. On top of that, it would offer broader and more evenly-distributed rapid transit coverage to a large quadrant of Scarborough. The current SRT alignment combined with a grade-separated Eglinton East doesn't work alongside a Scarb Subway IMO...too much RT in such a small area.
Lastly, and what originally got me to consider this line, is that it's a great compromise for those wanting a Sheppard Subway, and opposed to an SELRT.
Your numbers seem correct, and I'll probably do a further rough analysis for comparison of the options.