CRTC has an excellent web page that describes what costs may be involved in transferring a number over. It mostly involves cell phones as there may be costs involved due to being under contract. http://www.crtc.gc.ca/eng/phone/mobile/num.htm
If your are talking doing a landline to cable telephone or landline to cell as far as I know there are no additional charges beyond paying for service up to the last day you are with the existing provider.
The number porting system is managed by a 3rd party provider for all landline / cable telephone / cell companies in Canada (and the US) so from time to time there are some minor problems in porting a number so you may have to talk to your new provider to get it resolved.