Numbers scale exponentially tho. Let me put everything in USD base and then do a napkin comparison:

Montreal: - av salary: cad$70k -> $55k - av rent: cad$860 -> $656 - net: $47 128

SF: - av salary: $135k - av rent: $3.5k - net: $93 000

Taxes are very similar so let's keep them out.

Again obviously this is back of the napkin, and there are other factors, such as healthcare etc. but actually, unless you have a very specific situation (lots of kids, lots of health issues, etc) it still seems better to be in SF, COST WISE.

I'm not saying life is terrible in Montreal, I love Montreal :) But having done both, in the technologies sector, the difference is huge. If you work in another domain this is not true, but engineers are paid so much in the US, it's hard to beat.