One of the impromptu presenters at San Antonio Green Camp was Jim Champion of Texas Rainfall Catchment. Jim is located in San Antonio. He holds the designation Accredited Rainwater Professional from the American Rainwater Catchment Systems Association.
His website includes advice to those investigating catchment whether for drinking or for landscaping. he also includes a link to a Rainwater calculator which will help you figure out how much rain you can collect from your roof. He even includes a rough estimate of costs.
The range of services he offers includes turn-key systems all the way to help for homeowners wanting to build their own. He installs gray water systems as well.