I'm pro-fair-tax, but that number *is* on the fairtax.org website.
From the FAQ section of the website:
Is the 23% FairTax higher or lower when compared to the income taxes people pay today? Most people are paying that much or more today – much of it is just hidden from view. The income tax bracket most people fall into is 15 percent, and all wage earners pay 7.65 percent in payroll taxes. That’s 23 percent right there, without taking into account the 7.65 percent employer matching! On top of that, you have to add in all of the hidden taxes embedded in the price of everything you buy, from goods (averaging 22 percent) to services (averaging 25 percent).
Effective tax rates vs. stated tax rates Because the 23-percent FairTax would not be imposed on necessities, an individual spending $28,808 would pay an effective tax rate of only 15.6 percent, not 23 percent. That same individual will pay 17.3 percent of his or her income to federal taxes under current law.
Then they must of have gotten it from House Speaker Dennis Haster a few years ago when he was being interviewed about a book he wrote on his life on H&C on Foxnews. Mr. Hannity asked Mr. Haster to throw out a theoritical number on a Nation Retail Sales Tax and Haster stated "23%".