500 MW running for one second is simply 500 M joules.

Running for 24 hours it is:

500 M x 24 x 60 x 60 = 43,200,000 M joules = 43.2 T joules

If the tower is 200 meters high then you need to store this much water in one day:

43.2 T / (200 * 9.8) = 22 G kg

Since one cubic meter contains 1000 kg water, therefore it means:

22 G/1000 = 22 M cubic meter water

I assume that you need to store in water half the energy you need and the other half is used right away. Then you need 11 million cubic meter water.

If you have a dam as big as an approx cricket/football/hockey stadium (which I take as a square with each side one stadia or 200 meters) then you need a depth of these many meters:

22,000,000/(200 * 200) = 550 meters

I don't know the construction cost of that big a stadium. In Pakistan the cheapest brick is in villages, its size is 6" x 4" x 3" and come at a price of atleast Rs. 5 per piece. That is when its made on stoves that are heated by burning tires which is a very polluting way but we get along with that in village where air is very clean. We also have the advantage of having low prices. The labour of the brick factories hardly earn enough to feed themselves.

Since one brick is 24 square inches at maximum area and stadium is:

(200m x 550m x 4) + (200m x 200 m) = 440,000 + 40,000 = 480,000 square meter

Since there are about 40 inches in one meter therefore it means an area of:

480,000 * 40 * 40 = 768,000,000 cubic inches = 32,000,000 bricks = Rs. 160 million

This is when the dam's boundaries that is its walls and floor is just 3" thick. That obviously is not enough to hold a pressure of 550 m high water column at floor and 200 m long horizontal water pressure at walls. I assume we need atleast a ten ft thick wall. 10 ft = 120" = 40 walls in a row at walls and floor. The cost now is:

Rs. 160 million x 40 = Rs. 6.4 billion

I assume it to be Rs. 12.8 billion assuming 50% rise in expense each for labour cost and mortar cost.

Pakistanis on average consumes 220 watt energy and has a per capita gdp of $880. Gdp density is about 0.25 watts per dollar or 8 million joules per dollar. Since the cost of dam is Rs. 12.8 billion or 200 million dollars so it means an energy expense of:

200 M x 8 M = 1600 M M joules

The dam is supposed to provide storage for 250 mega watts for a duration of 30 years, the usual life time of dams. In that time the dam would store:

250 M x 30 x 365 x 86400 = 237,000 M M joules

The amount of money spent to make the storage place is just this much percent of the gdp gained in the process:

1600/237,000,000 x 100 = 0.675% (that is less than one percent)