I wouldn't have my kids pay for a trip until they were out of college and had a job.
Chances are if they are asking me to go on a trip with them, I'd just be happy they wanted to go with me

, and would just pay (as long as I also was interested in said trip).
Older DS is approaching 18 next month, and if he travels with his friends, he can pay for that (though I'd probably still help him with it).
DH and I decided years ago that we wanted them to focus on school, church activities/volunteering, and sports, which leave little time to work, with the exception of the summer when they lifeguard (and some tutoring for some pocket money), so they really don't have much cash at their disposal.
I suppose if they worked more instead of the other activities, I'd have different financial expectations of them so they weren't blowing it all on video games.