kicker

n. 1. One who, or that which, kicks. [ 1913 Webster ]

2. A fact, condition, or circumstance, sometimes concealed or not obvious, which reduces or eliminates the benefit of an apparently advantageous situation; a joker{ 5 }; as, under the Soviet system, bread was good and cheap, but the kicker was that you waited in line for hours to get any, if it was available. [ informal ] [ PJC ]

3. Hence: An unforeseen added expense or additional cost; as, the printer was cheap, but the special paper it needed was an expensive kicker. [ PJC ]