A newspaper advertisement does not create a legally binding agreement. The deal you reach with the car dealer and the contract you sign are the binding agreement. The fact that it looks like the advertisment was (1) could have been misleading; or (2) that it was mistyped as to be $20,000 under the sticker price for a car in that price range ought to be an indicator that there was a mistake. The other possibility is that this company has a history of conducting business this way and that could have legal repercussions. You haven't stated any facts that would indicate such.
You could consult with a lawyer in your area who handles auto sales contracts. Many of them offer free consultations.
No, and they won't. You would have to sue them and prove this was part of a pattern of fraudulent listings rather than an obvious mistake. The fact that you realize the mistake and that there are other listings with the correct price means you have no case.
So far, this is free to you. Until you pay a fee, I am not your lawyer and you are not my client, so you take any free advice at your sole risk. I am licensed in IL, MO, TX and am a Reg. Pat. Atty. so advice in any other jurisdiction is general advice and should be confirmed with an attorney licensed in that jurisdiction.