In the 1880s milk sold for $0.56 per quart or $2.24 per gallon which at the time was 0.112 oz of gold per gallon of milk.
To put that in perspective today, it would cost $190 for a gallon of milk.
So if you had a cow producing 1 gallon per day 10 months out of the year, it would have been the equivalent of having $57,000 / year in revenue today. (You have significant capital and labor costs gathering hay and water for just one cow not to mention the distribution costs and short shelf life).
At that same point in time, rent would cost $16 per month (equal to $1,360 today) and you could have paid your rent with the equivalent of less than 2 gallons of milk per week.
With prices like these it is no wonder that every family had their own cow which would have cost $70 dollars or 4.3 months of rent or $6,000 in today's dollars.