This is all well and good, but your illustration TOTALLY IGNORES payroll taxes of 7.65% (yes I know it has been lowered this year, but that's temporary) which means an additional $3,825 in taxes.
Moreover, Cain's plan also removes the employer's match of payroll taxes and, at least in theory, many employers would pass on at least a portion of this money to their employees. Now, you can say all you want that employers won't do this, but I can tell you that whenever I hire someone I am adding 10-12% to their compensation to cover payroll matching and other benefits.