The annual dues increase with income. If you make $20,000/year, you pay $60 (0.30% of your income). If you make $120,000/year, you pay $265 (0.22% of your income). If you make more money, your actual dues go up but your rate goes down. That's a regressive payment structure.
According to some statistics I found (U.S. Bureau of Labor), most anthropology professors have an annual salary between about $42,000 and $132,000, with a mean of around $80,000. Those $42k, $80k, and $132k levels are marked with vertical lines on the graph above. I'm presuming that range is where most of the AAA's dues payments are coming from.
I don't have any information on the distribution of salaries in collegiate anthropology or the AAA's annual budgeting needs, and I'm not accountant, so I can't demonstrate that this sort of proportional dues system would work as far as revenue. If it didn't work out that it would produce enough cash, however, the flat rate could just be incremented upwards until it did.
I know from personal experience that expenses associated with professional memberships impose a financial burden that can be difficult for those of us in the early stages of our careers (and/or without institutional support for conference travel, etc.) to absorb. Lowering the rates for the those least able to afford them would be a helpful thing to do, and it would probably increase the AAA membership. I joined last year because I had to interview at the meetings in Chicago. I'm considering not renewing my membership this year simply because of the expense.