Know-How Exchange : Research/Metrics

Looking for specific data points showing that affinity groups lower customer acquisition costs or marketing costs in general. I am attempting to make the argument that working through affinity groups such as alumni associations and other communities will help lower a company's marketing cost to reach customers in those communities as opposed to customers not affiliated with a group. An example could be a regular credit card vs. a branded/affinity card that might have a lower acquisition cost due to word of mouth, etc. Can anybody provide some specifics to help me? Thanks!!