I previously asked about the circumstances that allowed forgoing the "Prime Directive". The answer stated "All these people have violated the Prime Directive more than once: Kirk, Picard, Riker, Worf, Janeway, Tuvok, B'Elana." What is the penalty for violating it without Federation sanctioned justification?
Well, it's considered to be a Court Martial offense.
Usually the threat / warning is "You will face a court martial!" or something to that effect; I don't remember hearing anything about the additional concern of punishment; apparently the Court Martial by itself was a significant enough threat.
A court martial could result in a wide variety of punishments, ranging from Demotion in Rank, to Incarceration, all the way to Execution, if the crime merited it. Usually, the presiding judge had guidelines, but fairly wide authority to make the punishment fit the crime. A Technical violation with no impact might result in demotion for 30 days; an egregious violation with the perpetrator showing no remorse and expressing an intent to repeat the action could result in imprisonment, or even execution. (An example would be General Order 7; violation could result in a court martial followed by an execution. See the TOS re-worked pilot 'The Menagerie'.) (By the advent of TNG, I believe capital punishment had been eliminated, so death was no longer on the table, in a court marshal.)
I believe the minimum punishment for violation of the Prime Directive was a Dishonorable Discharge, but I have no canon backup; just a vague memory of someone saying that in one of the series.
Based on that list, the minimum penalty is "Nothing". Maximum penalty was discussed by Keith.
Picard was once grilled by a notable Judge over his violating the Prime Directive, but it didn't go further than being yelled at by soon-to-be-disgraced Judge over matter unrelated to her main Inquiry that Picard was interfering with.