I am trying the research the economics (ROI type-info) for web pages
for real-estate agents for a technical writing paper (comparative
analysis) at my local community college and for a friend who is a real
estate agent that needs a web site.

Where can I find some research information about return on investment for real
estate agents who paid for/use their own web site?

I am not seeking anecdotal stories about how to make a million $$$, if
only I buy this book....