Can be, and certainly is on paper, but in practice most everybody professionally involved with this problem knows that the groups at the most risk also tend to be among the most economically vulnerable. I know a lot of USians on Prep, none of whom pay out of pocket for it.
It’s heavily subsidized through public health channels, manufacturer rebate programs to help with high deductibles for insured people, and grant organizations like the PAN Foundation. Truvada has a generic now. Plus, in 2019 the federal government designated it as a preventative therapy that ~all insurance plans must cover without copay or deductible [0].
Now, it’s true that not everybody is insured (though people who are economically vulnerable tend to qualify for heavily subsidized insurance under the ACA or Medicaid). Still, the costs of Prep specifically in the US, while very high, don’t tend to be borne by the patients directly.