No you do not have to pay for Skype.
Skype is a VOIP application ...... a proprietary one. (VOIP => Voic Over Internet Protocol)
It uses its own proprietary codec/transport stream .... not quite sure now.
Google Talk also uses a 'non-standard' setup .... although they promised early on that they would support the VOIP standard - SIP.
So although both of those are VOIP applications, neither support the VOIP industry standard.
In the last year there have been 'gateway' applications that can connect between the Skype network and the rest of the world that uses SIP. I have not read anything about GoogleTalk integrating with the rest to date ..... although I have not been keeping up with the latest.
Simply put, there is an industry 'standard' - SIP - which is completely OS and application agnostic. It allows everybody who uses SIP, in whatever manner, to create a connection (call) between them, and that call is free when both are using SIP. Skype is a closed proprietary network, and GoogleTalk requires that you have a Google account also. In addition, GT requires that a PC/browser is used whereas there are solutions for Skype and SIP that do not have this requirement.
All can connect to the standard telephone network.
What is charged for this connection is determined by the VOIP provider and is a commercial decision on their part.
For instance, using SIP, I have free calls to USA landline and mobile phones, free landline calls to Ireland, UK, Australia and others as well as USA.
I dislike Skype because of its proprietary nature, and also dislike Google Talk because of its non-standard connection and its requirement to use a browser in a PC/smart phone.
Standard SIP, which has been used by industry for many many years, has no such requirements.
When Skype network and GT decide to support the SIP standard I will be more open to them.