@Wai Chan
I work for a local water department as a meter reader, I would say @Dan H. is pretty accurate with $100-$150 a unit (this of course depends on water rates and if you're billed in gallons or cubic feet). I would say low end usage for most people is around 1100 gallons a month per person without landscaping. Lawns and garden will drastically increase water usage.
I would also agree with @Seth Borman that getting low flow fixtures will also be a good first step; most departments and the SDCWA will offer rebates to assist with cost as well! I installed a free low flow toilet in my rental thanks to rebates a couple years ago.
Sub-metering can be great but there are factors to be aware of. You will not be able to shut an individual tenant off if they do not pay, you would need to make sure your lease is iron clad on how you handle non-payment of water. I like the idea of sub metering as an accounting and informational tool, however to keep it simple it wouldn't be my first step.
