Yes, about 50% of all trips are made with Metropasses. Of the remainder, 20% are made using Presto - a pre-paid fare card - and another 20% or so are made using tokens, which is another form of pre-paid fare media.
To say that only 50% are price-sensitive however is misleading. These people have paid money up front in order to allow them access to the system. Regardless of the medium, asking them to pay an additional fee to get on after they've already paid to purchase the fare media is going to make a lot of people re-think their choices.
You'd be better off just raising fares.
Dan
Toronto, Ont.