We use value delivered by the story as a primary measure of priority. Would it be more meaningful to consider Relative Value / Cost (relative affordability) as a formula for determining the priority of stories? Cost can be a relative measure of the complexity or effort for implementing and testing the story, determined by the development team (BAs and QAs) - in other words relative size of the story.
This relative affordability of a story gives us a measure of the worthiness of investing in a story. Judging purely based on value is inappropriate because the cost may be prohibitive.