The water division charges a flat rate based on the number of rooms and toilets/showers in the house, as that information is on file with the property tax assessor's records. If you ever add or remove a room or water fixture, you have to get a permit, which would get those upgrades reflected on the assessor's records.
Probably the greatest monetary cost you could cause the owner to incur is if they thought the toilet was faulty in some way due to the continuous flushing and had to call in a plumber.
Also I can't believe I'm seriously having a discussion about toilets on HN. Heh.
However, this is based on my own bill (total billed amount divided by total water used) -- a bit tricky since there are tiers and surcharges.
Also, we're under water restrictions due to the drought - water the lawn once a week, no washing your car in the driveway, etc.
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...
[2]: http://www.asknumbers.com/LiterToCubicFeet.aspx