They can have great value depending on the rarity of the error. If there were only a few with the error the value can be high to a collector. On the other hand, if there were thousands produced with the same error, the value will not be as great. A good site to research your coin is eBay, in the coin section.
Coins that have missing letters may have less value compared to those that have all letters printed correctly. However, the value of a coin varies depending on its rarity, demand and the year it was produced in. In case you have a coin with missing letters, you may take it to an antique store where you can find the exact value for it.