GTF, could you explain what you mean when you say the top half of the field subsidizes the bottem half?
Sure. It is likely a radically alternative way of viewing things, though. The faster half of a marathon race might finish in 3:30 (or 4:00 or whatever) or better while the bottom half would take 4:00-[however long the course is open] to finish. There is a cost, which can typically be broken down on a by-the-hour (or by-the-half-hour) basis, in keeping roads closed and staffed by police for a marathon course. The faster marathoners only need the roads closed for maybe 4:00 hours or so, at the very most, yet they pay just as much towards keeping the roads closed as those who take 5:00+ hours. In other words, all the <3:30 marathoners chip in to keep the roads closed for 3+ hours after they have already finished, even though they do not need the roads to be closed that long. Taking the idea to an extreme, the winner (unless his entry is comp'ed) subsidizes everyone who finishes behind him. This is not to suggest that there could or should be some sliding scale implemented to make fees more accurately reflect the amount each runner actually ends up using the course as that would be tough – nearly impossible – to do. It is merely an acknowledgment that all those back-of-the-packers, despite their entitled assertions that they help road racing to not just stay alive but thrive, actually would seem to get more than they give.