You don't have to pay any more, you can just use it as soon as it comes out of the box. You can however pay extra to get a few more services, this is usually in the form of a yearly subscription these services include :-
* Live Traffic
* Speed Cameras
* Points of Interest (Usually Free)
* Extra Voices (Celebrity / Novelty ETC)
* Fuel Prices
* Latest / Updated Maps
There are a few other bits, most people never even sync their satnav to their computer and are completely unaware of the other services.
Hope that helps.