Austin Texas collects 32 inches of rain a year. (See Green Building which has many stats about Austin's water usage & distribution. See also Austin Climate Summary and a Google search.)
One inch of rainfall on a 1000 square foot roof provides 562 gallons of water. (1in * 12in * 12in * 1000 * 2.54 * 2.54 * 2.54 (cm^3/in^3) * (1l/1000cm^3) *1.057 (qt/l) * (1 gal/4qt) == 623.6) (A conversion table is provided at Harvested Rainwater Guidelines) Thus, we can expect not quite 20 thousand gallons a year to come off the rain-gutters of a 'typical' urban home.
Installing a 1000 gallon collection system might cost around $1000 for the tank(s) and plumbing, if you do it yourself. This is assuming the water is to be used for landscape watering (rather than the far far more expensive purification for drinking water). (See e.g. Rainwater Collection over Texas for prices.)
In urban Austin, combined water and wastewater charges are graded, but are approximately $4 per 1000 gallons. Thus, to break even on construction costs, we'd need to refill that 1000 gallon tank 250 times. Given the rainfall in Austin, we might expect to do this 20 times a year, depending on the rainfall and watering patterns, the size of the collection area, etc. Thus, the tank might pay for itself in about 12 years, which is the same ballpark as it's service life (the tank will decay, depending on exposure to Sun/UV, etc.) We conclude: rainwater collection is approximately a break-even financial proposition for a 'typical' home and installation in urban/suburban Austin Texas.
(The economics is completely different in rural Texas, where problems with well-water complexly change the thinking & logic).