And I never mentioned "returns", that changes the subject. I'm not interested in changing the subject, which is, your statement that prices would go down. A preposterous thing when not identified as a projection. Crystal balls will be in short supply if this thing picks up steam. (An unlikely prospect)
You're right that future movements of prices can only be estimated based on experience. The future drop in interest rates under a consumption tax such as the Fair Tax has been studied by the Federal Reserve Bank of Kansas City. See:
http://www.fairtax.org/pdfs/interestrates.pdf