Welcome to FYC. Herby and Canarse have both made some great points, especially about doing your homework. For example, Lincoln has a lot of defensemen returning from last season, and another coming from Chicago in a trade. So they might have only 1 defenseman spot open at their main camp. Huge odds when you consider how many players attend these camps.
I've been following the Stars for a long time, and when I was running my blog, I would attend every camp. "Free agents" have an uphill battle to make the team. If I were to give an average, I'd probably say 1-2 players from each camp made the final roster as a free agent. I would suspect most USHL teams are around that number as well. But again, it depends on who is returning from the season before, and which players the team added via the two drafts.
So many of these teams that hold more than one camp during the summer are just looking at your son as way to make more money. That's it. However, at every Stars camp I've ever been to, there are tons of college, and junior scouts. So even if he doesn't make the team he's trying out for, he may catch the eye of a scout from another team or college. So in the end, you have to weigh the positives (exposure, chance at making the team) with the negatives (money grab).