truly there are too many variables to nail down an exact number, but here are some cost consierations based on my experience with a small herd of cattle-
first, I hope you have about 2 acres of excellent quality mixed grass and legume pasture that you can rotationally graze. if you do, and do a good job of rotating, this
should provide all your feed needs from March to November in Georgia.
second, you will need 1 full bale of
hay per day, top quality alfalfa or clover, during the winter months. dont try to skimp here, either on quantity or quality. figure 100 bales to give you a little margin of error. cost would depend on your region, likely $5-8 per bale.
third, basic nutritional supplements, at a bare minimum kelp and mineral salt. figure one block of salt and 20 lbs kelp. the kelp is the best health insurance you can possibly buy. $15 salt block plus $35 for the kelp.
forth, breeding the cow. AI is challenging with one cow unless you are good at identifying a cow in standing heat. Borrowing a bull is never as easy as it sounds, and adds definite feed and fencing costs. AI around here is $25 per try.
fifth, all the misc costs, such as vet care if needed, electric fencing upkeep, etc. This can be nearly zero if you are good, or better yet, if you are lucky.
so in total I would figure the cost of keeping a pasture-based cow/calf pair to be somewhere around $800 per year. Hopefully you are milking the cow, which should yield you around 600 gallons of milk in a low-intensivity grass based
dairy setup. The calf will be harverstable at 1 year of age, and should yield about 200 pounds of table meat at harvest. Raising cows is awesome, satisfying, and enriching. But it is not cheap, and things can definitely get expensive in a hurry. In the end, if things go well, $800 for 600 gallons of milk and 200 pounds of meat is great value for the homesteader.
hope that helps, feel free to ask any additional questions you may have. having a cow is the best thing a small farm can possibly do in my experience.