In a Service Provider (SP) network, routes for external destinations are distributed on iBGP sessions. A common practice is to make use of Route Reflectors (RR). Such a practice is more scalable in the number of iBGP sessions to be configured in a SP network than a full-mesh of iBGP sessions. However, it has been shown that RRs have a negative impact on the diversity of routes available in the network. This is an important issue as routers may not be able to quickly use an alternate route in case of a route failure. In a previous work, we proposed an algorithm to design iBGP session topologies with improved route diversity. In addition, we have shown that this is achieved with a low number of iBGP sessions, compared to a fullmesh. In this paper, we first propose additional metrics to evaluate the cost of a sparse iBGP topology ensuring NH diversity. The first new metric is a measure of the size of the BGP routing tables maintained in the SP network. The second and third metrics indicate the IGP and the peering costs of the solution, respectively. Then, we estimate the cost of our NH-diverse iBGP design algorithm by means of these metrics.