Well, while Rebtel can be free if you make what we call a smart call (where you ask your friend to hang up and call you back while you stay on the line) we usually say that the cost of Rebtel is the cost of a local call.
So, how does Rebtel's costs compare? Well, Björn did a comparison towards Skype a week back.
However, this blogger has gone further and compared the costs of Skype, Gozmo, Jajah, Rebtel and Truphone and found that Rebtel, while not always being the cheapest still is one of the most competitively priced services. With the added benefit of smart calls and the possibility to get calls completely free to the ones you call the most - I think that you'd quickly see that Rebtel will lower your phone bill the most.