For sports with consistant lighting, I toss about 10% due to focus issues and another 10-15% to missed shots (such as a player flying into the frame blocking my subject).
On average, I find roughly 25% are "printable moments" that I think are worthy of offering to parents and players. These are photos that are sharp and flattering of a player. The rest may be technically acceptable, but somthing "less than special".
However, for any given shoot, there may only be 1-2% that I truly like and only a few that wow me -- if I'm lucky.
For challenging events, like an ice shows under spot lights, I take more risks with a lot more shots. The keeper "rate" may drop, but the number of keepers generally increases. It just takes more work to get the pool of images that I'm after.
Keeper "rate" is a curious way to measure success. When I shoot sports for a team, my goal is to capture as many flattering images of each player as I can and to find perspectives of the event (including the fans) that are unique and memorable. I can increase my keeper "rate" by sticking with the safe shots. But, if I want more "wow" shots, I may have to experiment with more exposures. One example is capturing a controlled motion blur of a basketball player during a break away or in the air for a shot at the basket. These are tough to do consistanly and the more I try, the lower that keeper rate falls -- but, the odds of capturing that great shot go up.