Let's be optimistic & say you can somehow get batteries for $0.1/watt-hour, lower better. I just payed 5x that for some rc lipos, but let's be optimistic.

This is 10M$/15.66MWh/ft. The last is what makes this absurdly cheap. If those 5b gallons are raised up from the source only 1 foot, it'd be $0.64/watt-hour, more expensive.

But if you lift that water up 6 feet, it's down to $0.1/watt-hour, and suddenly you're at parity for the best anyone can expect. Lift that water up a 200 ft hill and you're down to $0.0032/watt-hour stored. Lifting a huge body of water is a colossal amount of work, it turns out, and thats what makes pumped hydro storage absurdly cost-effective.

Please also note the other comment. Apparently the new 5b gallon project is not $10m, it's unknown what it will cost, the $10m was for an old project. Even without knowing the cost of this new project, I expect the final figures will blow li-ion storage prices out of the water. For ex, if it it cost 10x more, $100m, but is up a 200ft hill, it's still $0.032/watt-hour. And there's really not a strong reason to stop at 200ft either; it's limited mostly by whatever the terrain offers (and how cost effectively one can engineer strong pumps & tubes. turns out we're pretty awesome at that.).