30 * 5 = 150L
If a family is away on vacation, you could theoretically flush for days.
24 * 150 = 3600L per day
In Seattle the water costs about 4.5 cents per cubic foot.[1] (If it costs 4.50 dollars for one CCF (which is 100 cubic feet) then I move the decimal place over two times.)
According to this[2] "1 Liter = 0.0353146667 Cubic Foot"
If that's the case then at 3600L per day you can flush 127.1328001 Cubic Feet of water.
127.1328001 * 4.5 = 572.0976005 cents
Round down to 572, divide by one hundred:
You can grief somebody for $5.72 a day in Seattle with this exploit. If you were to constantly flush all year it would cost you:
$5.72 * 365 = $2087.8
One way you could exploit this is if you obsessively watched out for the neighbors to see when they came and went and flushed when they aren't home.
[0]: http://www.inax-usa.com/download/catalog/pdf/0008-2.pdf (Page 18, flushing volume)
[1]: http://www.seattle.gov/util/MyServices/Rates/WaterRates/Resi...