Water is relatively expensive where I am and will be getting more so.
What I see on the web is relatively cheap.
I assumed you are in vicinity of Matawan, NJ.
Rates for Marlboro, NJ for example are $3.82 per 1000 gallons ( and unchanged from 2004:
http://www.gordonscornerwater.com/tariff.pdf )
What needs to be watered is my quarter acre (less the house square footage). About half grass, small veggie garden, and 20 or 30 shrubs. The monster oak trees take care of themselves as they have for the past hundred years.
For quick rain check I used
Average Weather for Matawan, NJ - Temperature and Precipitation
Looks like unless you have a spell of drought you don't need to water the grass at all (rule of thumb says one inch of water per week and most of the months are bringing around 4 inches of rain).
What setup do you have for watering today? In ground sprinklers? Garden hose attached sprinkler? Hand watering from the hose?
For veggie garden & delicate shrubs I would do drip, I would not worry about hardy shrubs.
If we assume 150 1GPH emitters (and most likely you will have less than a 100) for the garden and shrubs run for an hour every other day, you are talking only about 75 gallons per day on average.
2250 gallons per month at above prices is less than $9 per month additional cost.
Now if you want to sprinkle the "water thief" a.k.a. grass, if we assume 4000 square feet to water with 1 inch of water, it takes 2500 gallons to do it (ca $10 per watering).
As far a rain barrel collection - I would not bother with it in your situation.
Keep in mind that most likely you would need to buy a pump to distribute collected water.