Taking the ad to the Attorney General will not help YOU. This seems like false advertising so you might see a civil litigation attorney about a possible class action. I suspect this is not going to be worth the effort, however, for you. Part of the initial inquiry is whether you would be reasonable in relying on the error or were you trying to take unfair advantage of something that was an obvious misprint or clerical error. My guess is that's the case, but if not then seeing the lawyer will get more specific feedback. I expect you will be told you have little or no case.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.