In this paper we provide a new approach to estimate the link usage of multicast reliable transport protocols. It can be used as a benchmark to evaluate their scalability. We have chosen, as a case study, a multicast reliable transport protocol called RCCMP. It has been designed to be simple, scalable, reliable and TCP-friendly. The link cost is due to data, retransmitted data, and characteristic packets of multicast protocols such as control packets. For RCCMP, control packets are used to estimate the number of receivers involved in the session. Also, the reliability cost has to be added to the bandwidth consumption. Simulation results demonstrate that the proposed models are able to predict the bandwidth usage.