I don't think you really need to worry about buying bandwidth to support RWAN, I'd worry more about enough bandwidth to support your users and their applications. I really don't have an exact number on how much bandwidth RWAN needs, but I've seen no dropped packets or degradation of service on my 56Kb & T1 lines and thats with 2-4 RWAN/IPM collectors running, SNMP queries, WhatsUp Gold polls and 20-48 users.
HTH
Steve