It all depends on when your contract began and how much the device was. An easy way to check in by logging into your online account and checking the detailed PDF bill. It shows how much is owing on the device - all you need to do is add $50 to that amount and you have your cancellation fee.
Hi djgravel, Telus is a great mobile provider, if you're still planning on using a cell phone, you'll most likely need to start over again with someone else. We can't tell you exactly how much since we don't have access to your actual account. The answer provided by Dark_knight is accurate.