If you find an NADA entry for the EVS floor plan, it will include the slides that are standard in that floor plan. If you are looking at an entry for another model/floor plan, then all bets are off and not just because of the extra slide. It's a different model.
But the short answer is yes, an extra slide is worth more money (all other things being equal, of course)..
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL