No-shows is part of the business. Prepay only works if the event warrants prepay. I would prepay for a CQC event, I wouldn't prepay for a glorified skirmish. Prepay only works if their is a finite number of openings and a larger demand than there is openings. Why would people prepay when they know the event is wide open?
People have historically shown up last minute to big Ops. That's just how it is. The best course of action is to plan for a small number and expect more to show up. If you automatically assume you're going to get 400 or 500 players, you might end up disappointed. You can try charging more for the Op and weed out some of the flakes, but that might also turn around and bite you in the a$$.
No offense Rick, but your Ops most likely could not float a prepay. That's just not how they're built. Similarly, a Conflict Europe could not float a prepay. The only events that could charge prior to the event and get away with it would be a CQC tourney or a large-scale OP that has a high demand but only so many slots.
Just my 2c.