The volume of a sphere with radius r is given by the formula:

V = 4/3 * π * r^3

r = 4/2 = 2 m

V = (4/3) * π * (2)^3
V = (4/3) * π * 8
V = (32/3) * π

So (32/3) * π cubic meters is worth $500.

r = 12/ 2 = 6 m

V = (4/3) * π * (6)^3
V = (4/3) * π * 216
V = 288 * π

Let 'x' represent the cost of the tank with 12 m diameter.

(32/3) * π / 500 = 288 * π / x

x = 500*(288 * π / ((32/3) * π)) 

x = 500*27

x = $13,500

The cost to fill a tank 12 m in diameter is $13,500.