In Oklahoma City the estimated median household income in 2005 was reported as $37,375 by city-data.

I could not find a reliable percentage of taxes paid but I assume it is something.

In one consumer spending survey I checked housing cost at that approximate income level was nearly one third. Transportation cost which was largely fuel was about 19% and insurance, pension, and health costs was another 14%.

So by those figures nearly 2/3 of of the average household income, before taxes, goes for items that are not subject to sales tax.

If you assumed there was no tax at all and the entire 1/3 was subject to sales tax then that would be about $12,500. But that's likely high because of taxes and because there are still a lot of expenses not sujbect to sales tax.

It does seem likely to me that higher incomes would account for a larger relative percentage of sales tax but I couldn't find any studies. I suspect it has been studied.