A 1,000 gallon water tank weighs around 8,000 lbs when full.
A gallon of water weighs a bit over 8 lbs so assuming the 1,000 gallon water tank was completely full then it would weigh a bit over 8,000 lbs which is very heavy.
Then you add in the cost of the water tank itself which can vary depending on what the 1,000 gallon water tank is made of.
A plastic 1,000 gallon water tank weighs around 324 lbs and a 1,000 gallon metal water tank weighs as much as 1,400 lbs empty.
Put simply, a gallon of water weighs about 8.34 pounds (3.78 kilograms).
However, since gallons are a measure of volume rather than weight, the actual weight of a gallon of water will depend on temperature, surface gravity, height, and pressure.