why it cost C$85 for single - entry visa applying from Toronto, but only C$50 from Ottawa?

Because you have to pay service fee of Chinese Visa Application Service Center in Toronto, China consulate in Toronto doesn't accept China visa application from ordinary passport holders, if you don't hold a diplomatic, official/service passport, you have to submit application to the center in Toronto, besides the C$ 50 for single entry visa fee, the extra C$ 35 (service fee+13% HST) will be charged for the service of the center.

There is no Chinese Visa Application Service Center in Ottawa, you don't need to pay the service fee, just pay C$ 50 for the single entry visa.