Throwing around some rough numbers. Completing Sheppard from Downsview to STC would need about 13 km of subway, all in tunnel. Even at $250-million/km this would cost $3.3 billion. At $300-million per km this would be $3.9 billion; roughly that's about $3 billion more.
Also Phase 1 of the SELRT is 13 km; but about 8 km of this is east of where the Sheppard Subway would drop south to STC. So it's only 5 km of the SELRT that would be replaced by subway ... there's lot's of ways to get to $3 billion.
As much as 13 km of subway on Sheppard would be useful to a few people, I'd much prefer they continue instead with the 12 km of subway on Eglinton instead.
The $600 million extra for the B-D extension to STC is pretty close to what we had discussed in a thread earlier at some point. Sounds pretty reasonable. If any of the TC projects are getting re-examined, I'm glad it's that one. It's also the one that's most doable (from a practicality perspective).
Your numbers for Sheppard seem to be pretty accurate too. Although I still think doing a short extension to VP and then a BRT Light/BRT for the rest of Sheppard East is the optimal solution, as you wouldn't have to ask for extra funding.