200mL 2.5 yuan. Each milliliter is: 2.5/200=0.0 125 yuan.

3.80 yuan for 380mL. Each ml is 3.8/380=0.0 1.

1L 9.00 yuan. 9/ 1000=0.009 yuan per milliliter.

So it is more cost-effective to buy a third kind of milk.