We have the national geographic channel. There are no extra fees as it comes with the digital cable package. If you have basic cable, you may not get it.
Depending on who your provider is, they sell you a package with a specific amount of channels and then after a year they slowly yank a few channels from you. That ultimately causes you to want to upgrade. Its very deceiving!
It all depends on the package you have, most cable packages have National Geographic, you probably have a basic package and will probably have to upgrade to get National Geographic.