There's a lot you need to know before you can arrive at any monthly or annual dollar figure.
It seems you are concerned about the electric use only. That should have been something you got into before you bought a house with a well that was supplying 3 other houses but...
The true and complete cost is made up of the electric bill for the other three houses, all the cost for future pump replacement at some point and possible repair of the power cable down the well and maybe the drop pipe, and any control box replacement along with the pressure switch and pressure tank and gauge.
You should have received the balance in the account where the money is deposited from previous payments. How much is in it?
How old is the pump?
What size in hp and gpm is the pump and does it have a control box?
How old is the pressure tank, and what size is it?
How many people in each house and how many bathrooms and do any have large tubs or two person showers or any body sprays or over sized shower heads?
Do they have dish and clothes washers?
That info would give us an idea of how much water they might use per day, and an idea of when something is going to need repair or replacement and what it might cost if you did it yourself or hired a driller or pump guy, or a plumber that knew wells and pumps and is capable of pulling a pump or one that doesn't know wells and pumps; most plumbers don't.
You may feel you are in a position of power but... you have quite a responsibility here, these people have a history of getting water from your well and if they go to an attorney, you may find they have some power too; especially with our court system of today. Personally, I would never buy a property that has been feeding water to one or more other houses. Even if there was a legal contract or agreement in place. Time changes things and you may find you will be held responsible for the quality of water. I'd play this close to the chest and be a good neighbor and smile a lot. For all I know, there may be something in their deeds that require you to feed them water etc. etc.. Have you read your deed and understand everything in it? Also, they have had at least an oral contract at $15/yr for x years that you have now taken over. Maybe you thought that this was going to be a money making proposition, I say it has serious responsibility and you're out numbered 3 to 1.