Most seem to agree the sliding scale should be based on the income of the person receiving the therapy, rather than the person paying for the therapy or household income.

I was assuming Melania isn't employed/doesn't receive a salary as first lady, so with zero income and her husband paying for the therapy with his presidential income, she'd be eligible for the $80 discounted rate.

The single mother would not be eligible for the $80 rate because she earns more than $20,000. She'd have to pay more than Melania.

I didn't think sliding scales were intended to work that way, but maybe I am wrong.