The evolutionary cost of developing a particular trait is an attempt at quantifying how difficult the trait is to develop. Though the theory of evolution says organisms will tend to optimize fitness and do whatever it takes to survive, those predictions aren't the core of the theory and cannot be taken literally. Numerous times in nature a species will be confronted with a problem and never find a solution, instead going extinct; or they will take millions of years to exploit a resource which makes them prosperous. At other times these things happen quickly. Saying some adaptations have a higher cost than others is a way of quantifying or conceptualizing this difference.