How much can I collect?
A one-inch rain on 1000 square feet of collection surface will yield about 550 real world gallons of rainwater.
How much collection surface (roof) do I need?
Figure on at least 1500 square feet minimum for two people here in the Hill Country with our average rainfall of 32 inches a year.
How much do I need to store?
Allow about 5000 gallons storage capacity for every family member. This will get you through the worst of droughts.
How much does it cost?
A very, very ballpark figure is $10,000 for a 10,000-gallon whole-house system including storage tank, prefiltration, pump, secondary filtration and disinfection.
You can even read the first chapter of their how-to manual, Rainwater Collection for the Mechanically Challenged, for free.
(Thanks, Triple Pundit!)
These systems should be mandatory for new homes in dry places out West.
I've been using one of Rich's rainwater collection systems for about 6 years; it works great.
Depending on rain, of course. However, last year was so wet near Austin that I never bought water at all; all the water we used came from the sky.
If you use a rainwater collector to collect drinking water be sure to check on it often. Nothing like finding a dead thing floating in the tank to ruin your day even if your filter system and floride/ozone made it safe...
Also be careful that pure rainwater is actauly bad for you as is pure demineralized water from any source. While not deadly its not recomended for constant use.