What is the value of a rare 1978 nickel that has a nickel in the right corn?

Please don't jump to the conclusion that Different = Rare.

If you mean that your coin has part of the image of another nickel struck on it, it could be a mint error called a double strike. That happens when a coin blank doesn't slide out of the press in time and is struck again when the press comes down for the next coin. That might be worth a few dollars.

However, if there's some kind of special image or Wording or other inscription that isn't on all the other nickels in your pocket, you have a novelty item made by a private company. These companies take regular coins (usually cents, but sometimes nickels or quarters) and stamp special symbols on them - a Liberty bell, a picture of JFK, etc., and sell them at inflated prices as "collectibles". Problem is, there's very little aftermarket demand, so whatever you buy is worth little more than face value. To a strict coin collector, these are considered to be altered pieces and have no extra value.