Free here means free as in freedom, not free as in free beer. Non-free packages do not necessarily need to be purchased, but they put restriction on your freedom.
For example:
faac
(multiverse): this is an open-source library that lets you listen to FAAC audio files. The problem with the FAAC format is protected by patents. From Wikipedia:
Those intending to use this software module in hardware or software products are advised that this use may infringe existing patents.
nvidia-graphics-drivers-173
(restricted): this is one of the many nvidia drivers. The problem with them is that they are not open source, hence they can't be free software.
None of the packages in the restricted or multiverse repositories have a price.
By the way, Ubuntu offers software for purchase, but you can get it only with the Ubuntu Software Center, not with the official repositories.