a septic system put in for an SFR would be a concern.. and especially on that hillside.. were is your replacement field if your current field fails.. and tanks are generally 1200 gallons.. so I guess you will see how that plays out.. the septic issue would be a greater long term concern if I was a buyer.. renters are H###LL on throwing stuff in the toilet and down the drains.. :) 

It could be as small as a 275 gallon tank all the way up to a 1500 gallon tank .      

I think I save 4 gallons per flush.

Google says  Boston water runs $51-$56 per 1000 cubic feet (for some reason instead of gallons) and it also says about 7.5 gallons per cubic foot.

Jay I think that would be achieved by less government help in what we pay at the pump, true cost of gasoline is in the neighborhood of $15/gallon.

Is the price of gasoline now $6 a gallon?

This can be as simple as 1c bleach per 1 gallon water.  

I assume you are in the city so keep in mind PWSA is expensive and bills per 1000 gallons. 

If you do get stuck with a gallon or more of really good stuff in a large container that you've mostly emptied (like a quart of something left in a gallon bucket) that you can't use immediately, this technique will get indefinite storage out of it if done right.

Do you know how many gallons are they using?