Weeeellll... It all depends on what route you choose to take in care of your cow. Conventional or Natural? I have a Jersey, and through trial and error we've taken her from commercial dairy producer to family milk cow, and she is now as "natural" as we can manage to keep her. This is taking tinto consideration first and foremost the health of the cow. Second, though barely less important, the health of the products she produces is a very big deal.
I grew up raising dairy, first with goats then to cows(hopefully back to/adding goats again someday) so there are a lot of things about starting up that I take for granted... I've had years to develop my knack for it. As far as costs go... If you choose to go the "natural" route(I can hardly recommend "conventional")... If you have a few acres of good land, feed costs should be nominal, except for the winter. I live in the Midwest, and have around eight acres that are graze-able. With only two cows on that, I go through very little hay in a winter, as I have a lot of "standing hay" when winter rolls around. Last year I used one "round bale"(the big ones) which set me back $25. This year I'll probably go through two(my calf grew up

).
Grain is only required during lactation(some might argue that point, in either direction). Grain, actually, is VERY optional. It is a tricky thing and not fab for a ruminant animal to be ingesting. We only use it because it is cost effective... The best we can do with our budget. We avoid corn like the plague, as well as soy. We give our cow oats(high fiber=better for cow belly), which costs us about $30 a month. There is a whole gamut of great supplements(expensive) that you can add to that, which I tend to do in winter when her diet is less fresh. I wish I could afford to all year.
Our barn? Well, or cows bed down in an old garden shed we got with the property... 15x15ft maybe? It has a dirt floor we bed down with cheap sawdust from the local sawmill each winter. It works fine. We milk on an open-air concrete patio of a cinder-block building we have. We use a homemade stanchion my mom built for me. Labor was free(love my mom

), materials abut $120. That's optional, depending on your cow. My childhood cow stood happily, her head loosely tied to a pole.
Vet costs... Ack. Conventional vets are like conventional doctors. There are good as well as bad. Wring you dry types, as well as do you right types. I prefer to doctor my own cow, when I can. If you have your cow on a great diet, in a safe environment, vet fees will be few and far between... Just like people, I suppose. There are some really great books out there on natural cattle care, I would suggest purchasing as many as you can afford. Cross referencing is great.
Breeding... A cow's gestation is nine months. If you want a constant stream of calves, you can breed two months after her calf is born. You might have a friendly neighbor with suitable bull, in which case you walk her down the road(like I did as a kid). these days, you can call an AI(artificial insemination)tech and pick the stuff from your dream bull, and even get it sorted so you can pick the sex of your calf. Very nifty. I have no idea what that would cost in your neck of the woods. $50 and up around here. I chose bot to breed my cow right away for a few different reasons(ask if you care to, just trying to wrap this one up), and she is still milking 14 months later. I'm getting her bred next month... My step-dad has a stock trailer and is taking her to his neighbor's(where I bought her) to get bred by his Jersey bull.
There are tons of other things to consider. But the MOST IMPORTANT THING is will stress HUGELY is that you get a GOOD, HEALTHY, FRIENDLY, WELL TEMPERED cow. If you plan on hand milking then ask to see a demonstration of how well she behaves while it happens. If she isn't a perfectly sweet doll-baby cow, don't buy it for your first cow ever. They can be "fixed" sometimes, but it's hard work for somebody that knows how to do it. Ask a bazillion questions about her history. Take into account her age. Check her feet!!! If she had "big" rear feet, and the nail/toe has lots of horizontal ridging, chances are she has been overfed(BAD) her whole life, and they're trying to get rid of a liability. I don't suggest buying from a commercial dairy, not on your first cow. I bought my current cow such, and while I love her, I'll never do it again. Commercial dairies are NOT healthy places for cows, generally speaking. Check her udder. Acquaint yourselves with pictures of good Jersey udders on-line, and don't settle for anything less that marvelous. Ask about the diet they have had her on... How much grain they give her a day will be telling of her health. Based on my knowledge, anything over 10-15lbs a day is going to be detrimental to a cows health. Of course, YOU can feed her less, but she might have chronic issues due to daily foundering.
Boy, I can go on, on and on... But I have kids to tuck in. I'll be back, though.
