The cost of the violation will depend upon what state you're in. You'll likely get a notice in the mail within a couple of weeks and it will include the price for bail. I agree that police often do seem to discriminate against young people but, oftentimes, people charged with running a stop sign are guilty of doing so even though they feel certain they stopped. If you feel strongly that you are innocent, take your friend (and a parent if you're under 18) to court and state your case.