For example, the increase in customer satisfaction achieved by having a telephone system that allows you to route incoming customer calls to the exact location in your organization without requiring human intervention or the benefit of being able to seamlessly integrate home based workers into your telecommunications system are examples of benefits of a Hosted VoIP system that require a judgment call when determining value.
VoIP-News has a "Checklist: Adding Up the Return on IP Telephony Investment" that explains the Costs & Benefits of and investment in IP Telephony.