That is a great experiment. I gather you are just looking at expenditure as if there was no more potable water in your area.

Have you or will you attempt to find water resources and purify them to refill your stock while your using your BOB? In your area, situationally dependent, I think it would be highly important to find replenishment, either through nature or assistance.

I imagine that you would have tablets, iodine or some sort of filter in your kit at home or on the move.

I suppose lastly, and it may be off topic, but when a disaster is looming or just past and your staying at home, fill up your bathtub for an additional 30-50 gallons of useful water that would already be stored in your pipes and tank.