I love this game so much. I got so into it I didn't even notice half an hour had passed. I think it would be so rewarding to get indicators when you make $x dollars, like first 10k and there's a little celebration.
To make cows less OP maybe they could have a milking life cycle, so after a while they either die or you need to sell them as they don't make any more milk. Or something less depressing - run away (this can be at random so there is more risk of buying cow - they could run away before you milk them).