I just can't understand why they keep ignoring the obvious solution: finish the BD subway to Scarborough Centre, and then build LRT north to Malvern. It would cost the same as the existing RT refurbishment and extension by the TTC's own figures, and would dramatically improve the transit service to everybody in Scarborough. Is the allergy to subways so severe at the TTC?
The all-LRT solution will be the cheapest of the three (all-LRT, subway + LRT, or all-ICTS). The conversion of SRT to LRT will cost about 200 M more than the upgrade of SRT (according to the Dr. Soberman's report), but a much greater saving will be reached east of STC.
The subway (to STC) + LRT solution will cost more than the all-LRT one, as the new tunnel to STC has to be constructed instead of reusing the existing guideway. Nevertheless, one can argue that it might be more effective on the cost - benefit basis, due to extra development at the new subway stations, elimination of a transfer for many trips, and the release of the guideway land strip to build additional Stouffville GO tracks.
The all-ICTS solution is probably the worst of the three: most expensive, most imbalanced (in terms of the capacity-to-ridership rations south and east of STC), and least conductive to interlining.