Well, I'm not quite sure on price (as I wasn't the one buying anything for the goats) but if you are only speaking of vet expenses then they really aren't that much unless they get sick. They need wormed as needed (varying expense depending on climate, individual goat, and type of wormer), hooves trimmed (doesn't cost anything unless you pay a vet to do it for you), possibly treated for lice or other external parasites (again, on an as-needed basis depending on climate, individual, ect), and vaccinated (again, depending on where you live and what diseases are most prevalent in your area, but generally not that expensive). You may also ask for assistance with castrating the young or removing their horns. The most common reasons for needing a vet in an emergency would be for injury, difficult births, or unknown viral/bacterial infections.
The cost can vary so much that its hard to know, so asking around locally will give you an idea of what are the common diseases, parasites, and even local injuries/illness (rocky land, damp land, poisonous plants). Also, there are certain ways to lower expenses using other methods, but these do not always work as well. Again, asking around will get you a lot of opinions and idea on how to treat an ill goat and how to prevent disease. A draft-free, dry, safe shed with plenty of hay, a pasture to play and forage in, and prevention against hoof or parasite problems will keep most problems at bay.
Also, if you plan on getting milk from your does, keep in mind this will lead to other expenses (proper equipment) and possibly illnesses (infections in the udder, difficult births, sick kids). You will also have to feed a few extra mouths until the kids are old enough to go to a new home. Depending on if you take the kids from their mother early or milk the mother and let the kid continue to feed from her at the same time, which works if you only want milk for your family (they produce plenty), you may also have to pay for the proper formula for the young or seek out proper homes that can care for them (which is not always an easy task in some areas).
All in all, goats do not have to be expensive at all. It really depends on so many factors. A simple shed, some secure fencing, and a few make-shift feeders will do for a couple goats. You can even grow your own grass-hay (we did this). Research will help you decide which course of action is best for your individual needs.
(by the way, when it comes to animals, unexpected expenses are the common-place. The more prepared you are before you get them, the easier it will be. Starting simple may not be the best way to go about things if it leads to goats that constantly escape or injuries and infections)