It is only 50.00 CDN after Dec 01 ... not retroactive ... so make sure you either do it after Dec 01 or use the GOES site (see link above) and be charged 50 USD or you will be charged 80 CDN (up to Dec 01 2007).

Once you get your Nexus it is contingent upon you to update with the Nexus folks any changes to your documents - for instance you get a new driver's license (or renew it) or renew your passport, you must update the information with Nexus. In fact if I am not mistaken, it is now setup so that your Nexus is terminated just before your passport expires, so it is wise to renew your passport well in advance and pass that information along.