What kind of argument is this? An argument for futility... like why even bother building any subway/grade separated mass transit at all?
If SSE can't be built because of prohibitive high costs; then how in the heck could we ever hope to afford the DRL which has to cross the Don River 4 times, 16 lanes of the 401, several rail corridors, navigate through tall building foundations, the PATH system and some of the oldest utilities infrastructure in the City?
Surely DRL Long will cost upwards of $15-$20 billion if the SSE is costing in the ballpark price range you speak of. The answer is to stop pitting neighbourhood or community or borough against one another and to recognize that mass transit is needed ALL over the City, not just in one part.
A subway to Scarborough Centre will still be far more practical for most traveling through Scarborough than taking the York Mills bus across to Don Mills and boarding the subway from there. DRL has its purpose, but so does the Scarborough subway.