Would a coach returning an email invite to the camp be considered a true invite or just an generic email to get you there for money?
Many times, when you write to a coach showing interest, they will send a camp invite. It's pretty routine for them to do so. Is it an invite? Of course it is, but unless it's personalized, it doesn't really mean a lot. Players should do their best to establish some sort of communication with the coach, before attending a camp. You want to be on his radar. like others, I believe you have nothing to lose by attending a camp of a school you are interested in. However, I would not break the bank to do so at a school far away unless there was some genuine articulated interest by the coaches. But if it's a local camp, and the price is right (usually between $100-$250), then it might be a good opportunity to be exposed to a college coaching staff, facilities, coaching, etc. There are schools (mostly D3s) that do end up with players on their team that attended their camp. It just depends on the school. Many times, they will tell you how many players were signed from last year's camp.
If your son sends emails to many teams, expect many camp invites. Yes, they are money makers for the schools, but that doesn't mean they are not good experiences for the boys. Hope this helps.