According to the information I've received from a tournament director - the $300 applies PER EVENT. So if you've won $305 in a single tournament, you have to report your average for that event.
Last year, I won $375 in one tournament, $325 in another, and $425 in another. I was required to report my averages for each of those events. The tournament director of the tournament I was intering took those averages, averaged them, and split the difference between that and the tournament average of the tournament assn. I was entering. It resulted in a rerate of about 9 pins.
Does that make sense?