How low ? well, real low here is an example - If you are using a conventional voice-over-Internet-Protocol calling plan or Skype Call service a call to Argentina - Buenos Aires per min would cost you $ 0.21 where as the same call via Google costs $0.02. You can see the whole comparision chart at skype and the call rates from Google.
Rates from Skype Vs Rates from Google (or the detailed rate list)
If you have a Google Voice account, the other person will see that number in their caller ID. You can also answer their calls from within Gmail; an incoming call will generate an alert in the bottom right corner of the Gmail window, which you can click to answer.
If you don't have a Voice account, the other party will see a special number Google has set up, 760-705-8888. Calls to that yield a message advertising the new Gmail feature.
So go ahead and try the free phone calling option from Goolge and do share your views below