The answer is: "Depends". Though if your business model is immoral when applied to poor folks, it's still immoral when applied to rich folks.
The markup on drug store cotton balls is shockingly high, like 1000%. Nobody who buys cotton balls cares though, because the $2 per bag they end up costing is a very small fraction of their income.
But take that to a developing country, and that $2 is completely unaffordable. You would be forcing people in need of sterile cotton swabs to pay a day's or a week's earnings, when their manufacture costs a few pennies.
I don't think the former example is unethical (indeed, the cost of a bag of cotton balls (in America) is probably far below what the market could truly bear). That stupendously high markup is fine because it still keeps the final price easily affordable.
In the latter example, that same markup makes cotton balls all but impossible to buy, when even a "normal" retail mark up of 25% would have kept the price in the cents and therefore at least approachable. I'd call that unethical.
If setting prices too high is unethical, then is refusing to sell at all also unethical?
And if it is unethical, should it be mandatory to sell any good at a reasonable prices? Or only a class of goods deemed "essential"?
Similarly, I believe it immoral to sell a necessary good at exorbitant prices, but fine to sell luxury goods at whatever price you like. Of course, the question follows: What counts as "necessary"? I don't have an easy answer for that one :)