However, this means that before-tax prices will generally not fall enough to offset the FairTax. They will fall a bit (as producers are relieved of administrative overhead costs, and forced by competition to pass the savings along), but not by anywhere near 20%.
And of course, all of this is just talking about the immediate effects. By the same Jorgensen model, the economic growth in the country would be astounding (9% growth), and as the saying goes "a rising tide lifts all boats".
You haven't considered the business income tax costs that are presently embedded into prices as increased prices caused solely by the tax system.
With the removal of these tax costs (which include not only business taxes but also compliance costs) prices will drop a good noticeably. Your claim of "not anywhere near 20%" is merely that - a claim which is backed by no facts. Time will tell as more information becomes available.