A doubled mint mark is only worth a few dollars more than the average price of a cent with nothing wrong, if that's what you mean, or if it's multiple letters, etc., it depends on the classification type of errors it is graded at. It also has to do with the grade of the coin(circulated or uncirculated). Grading is everything that determines the value, even with D/D coins. A good reference point to use is the PGCS or NGC websites. NGC gives you a more complete grading value for standard coins and PGCS gives you the most active coins being sold with some of the most recognized error coin in any specific year. For the most part , any non-specific year, you can get an idea of what the prices are from EBAY. Just type in double die LIncoln Cents, and you will find a number of types listed, graded and ungraded, be it a set price or follow some on auctions. Hope this help. Have a good one!