This paper presents an approach to estimate the cost of the PIM-DM protocol in terms of the number of packets, both for data and control traffic. The proposed approach assumes a loop-free network topology and that all links have equal parameters. Although restrictive at a first glance, the results show a good performance in simulated real networks when mean values for the parameters are used. The expressions are deduced from the protocol functioning, overcoming limitations and approximations of previously published works.