Each milliliter is the total amount of money ÷ the total amount of water = 1.5÷250.

=0.006 (yuan/ml)

400 ml 2.2 yuan: 2.2÷400=0.0055 (yuan/ml)

500 ml didn't give the total price!

If the unit price of 500 ml of water is the same as that of 250 ml of water, then 3 yuan =0.006 yuan/ml is needed.

If the unit price is 400 ml, it needs 2.75 yuan per ml =0.006875≈0.007 yuan /ml.