To clarify, there would be some significant net benefit from getting rid of red tape and economic inefficiencies driven by the current tax code, but obviously not enough to fund the government from out of nowhere. The most reasonable estimates I've seen are that pre-FairTax prices would fall about 5%, thus producing a corresponding gain in the average person's purchasing power.
Actually, my numbers in post 72 are as good as any, and I estimate about a 7% fall counting all the taxes saved by businesses and a reasonable amount of savings from compliance. Anybody claiming more than 10% is smoking something.