I have kept meticulous records since I purchased my cow - recording all expenses including the cost of advertising to find the cow. I have also recorded the costs of purchasing expensive one-time items like my electric milker ($700) and the cost of the cow ($1,200). I include the costs of purchasing cheese-making equipment, ingredients, etc. also.
Over the four years that I have owned my cow, I have spent a total of $6,430.71. I have brought in $3,266.10 by selling 3 calves, cheese, milk, butter, etc. I conservatively estimate that on average I have collected 3 gallons of milk per day for 9 months each year. That comes out to about 810 gallons of milk per year. (I think the actual average gallons/day may be one or two gallons more than my estimate).
So, the cost per gallon of milk comes out to just under $1.00 based on my records and estimates. Obviously, there are difficult-to-quantify costs such as the cost of owning land (I have just under 1 acre), and the time I spend milking, storing milk, cleaning, etc.
Similarly, there are difficult-to-quantify financial benefits such as money saved on purchasing milk, cheese, butter, cream, etc. from the store.
All of that aside, I think that as long as you enjoy spending the time required it does make financial sense to own a cow - something I never suspected when I first purchased her. The time commitment is about 1 hour every day to milk, feed, and clean up. When I switch to once-a-day milking, that time will be cut nearly in half. The way I look at it is that some people have a daily exercise routine at the gym or wherever; I have a daily exercise routine in the barn. My routine results in a lot of good milk to use every day and satisfaction from producing a lot of my own food - not to mention the fun I have making cheese.
So are you going to get a cow now?