There's a few other things to take into account when referencing the guys who make the most money, and that's the affect the Olympics have on the average joe every 4 years.
It's the one time that people seem to care about track and field, and you finally have a few prospects that are pretty marketable in events that have been dominated by the east Africans. Rupp, Solinski, Wheating, Centro, etc...
You pay the guys who have the highest probably of giving your brand the most exposure it can get... The above mentioned guys are probably all worth an Olympic "interest" piece giving Nike more exposure than just to the elite running world. Here's your underdog average white guys to the rest of the world... they're like Brian Sell's for the general population, but instead of being the ultimate fan boy jerk off material for the untalented in the States, you've got it on the world stage.
It's sad that more people can't make a living off of running, but the market speaks for itself. Football players and Baseball players all have a few things in common that help their values go up... first is a players union and second, they have a league that's worth billions annually.
Example, I'm from Wisconsin, I love the badgers, the brewers and the packers... when it comes to track I root for.... America? yea, once every 4 years... other than that, track is a completely uninteresting series of uninteresting athletes running times that the general population, even a majority of the running population, have no context to appreciate what they just saw.
We just have to accept the fact that track will never be important until we find a way to make it entertaining. Until then, expect all the money to go to the few people who have an opportunity to make some impression on the average person, which is pretty much only possible in an Olympic year.