Cows can average around $3000 to $4000. Yearlings as less expensive. You can find an active cattle market if you live in the right areas. Prices may vary depending on locale. Asking a local farmer or cattle farmer is a good idea, as they can lead you to contact sources to buy cows. Another place to look is a cattle exchange.
In terms of supporting cows, the general rule of thumb is one cow per roughly 2 acres. If you are buying cows as a form of investment, note that as population increases, there will be more demand for livestock meat.
Note that since male calves do not produce milk, they are usually raised and slaughtered for beef.