OK, I organise lots of secondary school trips.
The 'first come' penalising parents on low income is a thing you can work around by putting a token deposit on the the online system (we use ParentPay), to be followed up fairly swiftly with another payment OR an email/call to the Finance Office asking for a more flexible arrangement - all explained on the letter.
Letters of interest are sent out explaining when the trip will be going 'live' on ParentPay. Students in receipt of Pupil Premium are offered a separate letter explaining they get 50% discount. The letter goes out a few days before the trip goes on ParentPay - students whose parent/guardian will not be able to find even the token deposit on that date are offered the opportunity to contact Finance Office in advance. It doesn't happen often, but we've set the system to accept £1 deposits before now...
I have colleagues who prefer to organise a draw, but IME it causes further rancour. You're going to get someone's parent screeching at you down the phone whatever you do wrt over-subscribed trips, but it's harder to argue with a 'first come' system than it is with a draw that parents generally suspect you of fiddling. As it happens we don't - we're very upfront with parents if their child is not being considered for a place!
As for running over subscribed trips in the first place...we generally do fit everyone in, by means of careful booking & sometimes waiting lists. But it sometimes has to be called before you get the deposits in - you ask for expressions of interest in a trip & get 45 responses. So - knowing some will not actually be able to commit - you book it for 40 kids & 4 members of staff. If another 10 kids then want to go (& you can strongarm a fifth colleague!) the travel company may be able to increase the numbers - they certainly will if they can, to up their profits. However, what you can't do is book, say, 43 on the trip - the company only gives you one staff place per 10 kids so your staff/student ratios will be buggered.
Also, I run an annual 2 day trip for 90 kids. There are 180 in the year group & this one's always over subscribed. But to meet demand I'd have to book 135 (so it'd be 3 coaches not 2), which then means 14 staff members - imagine the supply bill on that one. Plus muggins here would have to be in charge of them all walking across London, in a busy restaurant & theatre, in a chain hotel, at a large theme park & at several service stations. Frankly, me taking responsibility for one coachload & my mate taking one other is as much as my nerves can stand!
So - well, it's logistics, I'm afraid - that & a diminishing pool of teaching staff willing to get involved.
Anyway, I think YABslightyU, OP, but I think you've agreed that already.
. But I'd avoid the situation in the first place if it were my trip by going 'first served' so you either got them both booked on, or neither.