Having recently helped my mother in law get health insurance, I can give you estimate numbers! In Maryland at least.
My MIL is a little under 60, a small business co-owner with no group coverage options, and she now has a very good plan through BCBS as an individual. It is pricey at ~900/month, but has no deductible, 10% coinsurance, 1800 annual in-network out of pocket limit, and 3600 annual out-network. This is an individual plan with no government subsidy (since she makes too much) and nothing to do with medicaid or medicare.
There were cheaper options for her, one at ~$400/mo, another at ~750, with corresponding higher deductibles and coinsurance.
My MIL never had insurance in this country prior to last year. Thanks to ACA laws she was able to get full coverage with no exclusions or anything despite lack of prior coverage. I don't know anything about what would happen with different immigration statuses but I never remember seeing anything about ins. plans being restricted to citizens only in our research. Your MIL simply will not qualify for the gov subsidy. Unless she reports low interest/dividend income to the IRS maybe? Does she have reportable retirement income from her country? Might be worth looking into laws about filing income as a resident.