What is the value of a 1970 D nickel with a mint error reverse?

It depends on the severity of the error, the condition of the coin, the type of error, whether or not the error is a recognized "Cherry Pickers" variety and last but not least how well one can see that error with the naked eye, as opposed to needing help to see it with a magnifying glass. If the coin is off-center by 15% or more and in Excellent condition I would give an estimate of somewhere between 10 to 20 bucks on eBay... a good way to price your error is to go to eBay, go to the "coins" section, go to the "errors" section in coins and to see if an example of your type of error is listed in the "completed auctions" section to see what the final price of an item is worth... I would not make a judgement from a currently ongoing auction since many people bid right at the last few seconds.