I agree with you that double triggers are an advantage on a hunting gun (probably not on a target gun). I understood your question to be about price and not some other measure of "value". If you mean price in money, I am fairly confident that double triggers reduce the average price that the gun will bring in the market. While the three of us agree that double triggers are advantageous, most buyers disagree.
By the way, note there are 3 S56E guns for sale on the national internet sites, one as low as $500 in decent but not great condition, that remain unsold.