If you’re a US citizen, then you are required to sign up for Medicare. Why aren’t you doing that? In addition to Medicare, get a supplement for additional coverage. It won’t cost you anywhere near $750.
I didn’t sign up for Medicare because I wasn’t a resident of the US, but will sign up in June when I return.
It certainly won't be $750, but it won't be cheap. To get complete coverage, you start by paying a bit over $100 a month for Medicare. It's deducted from any Social Security you might get. A "Medigap' policy - a policy from a private insurance company that covers charges that Medicare doesn't cover - can cost upwards of $200 a month. Then, most experts also recommend a private drug coverage program. Even if you don't need this now, having it prevents major cost increases in the event you require major drug costs later in life.
The combination of these can approach half of the $750 mentioned, all for our "free" government insurance.