Matt,
First is going to be distance traveled and prize money. I'll look at what contests I could go close to in the same time period before I will drive a long distance to another contest. Given that you're looking at July and its in Oklahoma, thats a busy time around the KC area where I can tend to pick from a large pool of contests to go to. Yes, the prize money might be bigger for your contest, but if I'm looking at a larger entry fee and a much higher cost of getting there, it might not offset enough.
From there its looking at what the contest offers. I prefer 20 x 30 spots to start with - it lets a smaller utility trailer setup in the space, have space to setup cookers and still have a little room left over to sit / hang out in. Basic power is almost a must - up to 20 amps on your own breaker. If you're sharing a breaker, you'll see problems with flipped breakers too often. An upcharge to 30 AMP isn't a problem - a $25 upcharge for that is still cheaper than running your generator on current gas prices.
1 or 2 bags of free ice are nice, and then a reasonable charge for ice. $2, you bet. $4 bucks for a small bag? Excuse me? I realize you're trying to make money for a cause, just don't expect to bleed your contestants dry. Also have water easily available or be willing to have carts run us to the water faucet. Few things worse than having to cart heavy water jugs a long distance.
From there, the extra curricular activities aren't usually a big draw - it can be enough to get there, get prep done, grab some dinner and catch-up with friends. Let the Friday night activities be the reason the public is drawn in. Also, don't expect teams to stay after awards on Saturday. Most of us can have long drives / things to get done, and for us to have to stay to keep the public there, etc., will cause cooks to not come to the contest due to delays in their travel schedule.
Also, July in Oklahoma is hot. I did Art of BBQ in July in 2009 and it was 115 in the shade. Did I mention hot? I would consider looking at some other months where its not as hot and possibly not as "crowded" for contests (March, April, May, November). You could get a bigger draw from cooks looking to cook "one more" or kick the year off. Just a thought.