This is UK prices, but I use Pay as you Go on Three at 1p a mb.
I tried FaceTime and Skype recently in London... I gave up with FaceTime because it was too laggy (needs high bandwidth)...
Whilst I was on it Facetime used maybe 3-4mb a minute, which is around 4p a minute.
Skype was way less though - worked far better, and after 45 minutes it had used 27mb... so that's 27p for nearly an hour of video calling!
Since you're looking at audio only... well, I imagine it'd be 3/4 of that?