Cows are big source of income. Milk produced by cows are costly. So if you sell milk of cows then you can become rich easily.
For that you have to buy good cows. Hybrid cows are now available which generates literes of milk.
Cows are easy to tame and they are human friendly. They also need only a little food. Usually the waste from kitchen like vegetable waste can be given to cow as food.
So cow becomes a good source of income for your family. So its economical to buy a cow or a number of cows. Hope you understood how to earn by buying cows.