California Code, Business and Professions Code - BPC § 17501

For the purpose of this article the worth or value of any thing advertised is the prevailing market price, wholesale if the offer is at wholesale, retail if the offer is at retail, at the time of publication of such advertisement in the locality wherein the advertisement is published.

No price shall be advertised as a former price of any advertised thing, unless the alleged former price was the prevailing market price as above defined within three months next immediately preceding the publication of the advertisement or unless the date when the alleged former price did prevail is clearly, exactly and conspicuously stated in the advertisement.