I frequently fly into Toronto from foreign countries (including the US), and it has always been my experience that baggage carts are free of charge in the pre-inspection areas where people collect their checked bags. There is no charge for exactly the reason that Lucy mentions: visitors can't be expected to have (what to them are) foreign coins.

This is also my typical experience in other airports around the world. In those where they charge for the use of baggage carts, this only occurs in the domestic areas. Passengers inbound from foreign countries have access to free carts in the pre-customs areas.

Doug (San Francisco)