skype is closed source, as they use a means of connecting that was, and may still be, encrypted end-to-end and that they wanted to keep proprietary. i am not sure if gizmoproject is closed source or not, but ekiga is fully open source.

the cost of making phone calls comes into play when calling phones that are not IP based, i.e. cell/mobile phones or traditional landlines. the carrier of those types of phones run a "use-my-bridge, then-pay-my-fee" network, and in order to access/ring up on their network, you have to pay. thats really the only time costs come into play for VoIP calls (aside from the needed broadband connection).