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Is this historical? I don't recall too many cases of this sort during this period...most regicides happen after a monarch is deposed (which means "killed" in eu terms), as part of a coup (Gustav III of Sweden, Paul of Russia) or by the hands of religious zealots (Henry III and Henry IV of France). It seems monarchs were quite well-protected from foreign assassins during this period...or simply no one bothered taking such a risk.
So yeah, if you intend to do it - and given the utmost importance of a monarch and his stats in this game - it should probably cost everything and still might have a significant chance of failure.