The cost of a dairy or milk cow ranges from $900.00 to $3,000.00 per cow depending on location and age of the cow.
Plus then there's the cost of caring for the cow so although fresh milk from a cow is great tasting and more healthy for you it's certainly cheaper to just buy the milk in stores than it is to raise the cow for milk.
But then again you could eventually slaughter the cow for it's meat so I guess you won't be out too much but you would have to pay for slaughter of a cow as well if you don't have a way or knowledge on how to slaughter the cow yourself.