The iPhone is sold as carrier locked only with a number of carriers in Canada now - formally this was with Rogers/Fido only.
This depends on the carrier you will be using the iPhone with, with the iPhone sold as carrier locked with that carrier. If a particular carrier for the iPhone that you want to use does not offer the iPhone on a pay-as-you-go basis, they won't sell the iPhone without a contract.
This link includes the carriers for the iPhone in Canada. You need to choose the carrier you want to use with the iPhone first followed by contacting the carrier to determine if the carrier sells the iPhone without a contract and on a pay-as-you-go basis. If so, this will be the most expensive price for the iPhone. If there is an Apple store nearby, you can call the Apple store after choosing which carrier you want to use the iPhone with first.
http://www.apple.com/ca/iphone/buy/