Most universities (that admit you with funding) will cover the cost of your insurance. Usually, schools will have a type of insurance plan for graduate students (not just in mathematics, but other departments as well). This is a subsidized kind of healthcare. If the program admits you with funding, you will either pay some kind of premium (usually around 2,000 USD a year--maybe more, maybe less), or the department will cover the cost of that premium. In addition to this, you may or may not have some kind of co-pay (a flat fee) for a specific type of medical service or exam. The rest of the cost is covered by the insurance company. The type of coverage varies for each university. Meaning, some services are offered and some are not. Usually, basic appointments to a university clinic, x-rays, etc are all covered. Something like dental care may not be. You might have to pay a deductible for an expensive surgery or an emergency room visit.
It really doesn't matter if you're international or not for most places. Mostly, it matters if you get funding or not--which can be more competitive depending on whether or not the institution is private or public. If or when you are admitted by universities, I strongly recommend that you ask about the specific details about the insurance policy.