I suppose if you're buying it online instead of locally, that's an option. If I were to buy it from a local store, though, it's a lot more expensive than that--$16 will get you 14-30 doses, even with a cheap brand. And that's assuming you can find the cheap brands in stock, which is often difficult.
I don't think it's reasonable to expect that people in financially difficult situations are able to order online. These are often people who can only pay with cash, food stamps, or other methods that Amazon won't accept. If you're shopping on Amazon, you're probably in a position whereby you have a bit more flexibility in your life.
I know that in Massachusetts (United States), you can see a PCP, get blood work, and get a Vitamin D prescription for free if you don't have sufficient income or insurance. I have friends and neighbors who don't have jobs right now; that's how they get their healthcare. Of course, they're lucky: from what I hear, it's certainly not like that in most of the US, let alone the rest of the world.
Regardless, I don't think it's constructive to insist that self-medicating with Vitamin D should be a priority for people in such a financial position. There are other priorities. If your life is comfortable enough that you can worry about supplements, you probably have enough money to get better insurance or get proper blood tests.
Edit: I don't understand the downvotes. There are an awful lot of people for whom buying medications on Amazon just isn't an option. They don't have and can't get credit cards or bank accounts. They don't have a stable place to live or a way to receive packages. "I don't understand why these people can't afford X; I can order it online for just Y cents" is entitled and dismissive: that's not the reality for many people, especially in the US.
Meanwhile, a not-insignificant percentage of these people live in places where they can, in fact, see a doctor, get blood tests, and get basic prescriptions for free (money). They often can't take advantage of these benefits--not because of the costs, but because of the time (taking a day off from work? Not an option.), bureaucratic red tape (paperwork, call random people and wait on hold for hours), and lack of awareness (how do you know these services exist, especially if you don't speak English or even know to look for them?).