I think if people are moaning that they wealthier kids are missing out, then their has to be a blanket rule of all kids get go in trips full stop regardless of money.
It's not fair if poorer kids are missing out on some trips because they can't afford it, it the wealthier parents are happy for that to happen only to moan about their kids missing out because, well they can throw money at it and can't accept them being penalised for "having too much" however that was determined.
I'm not articulating this very well.
If you are going to be ok with kids being penalised and missing out on trips, you need to accept it can work both ways, having too much or having too little.
In my opinion, a school trip should be accessible to all, but sadly that's not the world we live in.