For the local tryouts, the scouting/organizing is farmed out to local travel ball clubs. They decide who is going to make the trip to Cary.
The entire trip to Cary for the "second" round is all out of your pocket.
Unless your son has always been the most dominant player (by a large margin) in every tournament and league he has played in, the chances of making the development team are near zero, so don't go for that.
Once their they have USA baseball scouts that watch every game and take notes, but in reality I think they know who they want to watch and everyone else is just there to provide bodies for games. I'm sure there is the rare case of someone they don't know that hits several HR's or pitches 10+ mph faster than everyone else in the age group, but for the most part position players just aren't getting much of a look. Just a matter of too many kids there for scouts to sit in chairs for 12 hours a day and really pay attention to all the players....that would be an impossible job.