if you read some of the method in the how the stats were measured in American football (in the links that I provided), you would see that they did not just take measurement of how many yards were gained and divide by how many times the player got the ball. A football eye watched the play a few times in order to determine the value that each player contributed to the play.
of the top of my head a model would have to involve some type of scale that was able to account for the difficulty of pass attempted, the other options available (opportunity cost), the level of pressure the passer was under, where on the pitch the passer was, where on the pitch the pass went, how far the ball traveled, controlability of the pass, the danger creation of the pass. Then rate all of these on scales from -10 to +10. Then figure out a way to weight each category apropriately to come up with an overall rating for each individual pass, then this stat could be used in many ways both in average score per pass and total score created through passes
Like I said it would take a long time and it would have to be done by the same set of eyes to keep things consistent, and it would never be perfect, but at the same time, nobody will ever draw a perfect circle (even with a compass, (if you go by the most strict mathematical principle of what a "circle" is)), so perfection is not the goal, the goal is to represent the value of player by a universal measure using numbers