Contract to valid or void

If you SIGNED it, then your stuck. This is exactly what scammy sleazebucket dealers do, advertise a false price, get you to sign a contract without thoroughly reading it, and laugh all the way to the bank. Whatever price is on the contract is the one you legally agreed to pay. Doesn't matter if their ad is bogus. Check your states Attorney General's office for help in dealing with shady dealers. New cars at least have a 30 day lemon law but you'll have to see what's available for used cars.