Interesting. As many many PPs have said, circumstances are relative.
Your family genuinely going without- food, bills, decent clothes, etc - make a smaller donation or speak to school about difficulty paying.
Feeling that because you have 3 DC life is expensive and the option of "donation" means you could save a bit of cash to spend on other nice-to-haves not needs - unreasonable. You're prioritising your family at the direct expense of others - not enough money in pot, trip cancelled and for some kids school trips are the only chance to see wider opportunities.
Our PTA subsidises trips for all parents. Mostly, no one appreciates it. If we don't make the fundraising target, perhaps this is the first to go & set up a hardship fund instead.
People complain but don't realise how good they currently have it. It would be a strange school that isn't acutely aware of trying to keep costs down for parents whilst still maximising educational value - if you can afford the trip cost in full, please pay. Otherwise the inevitable end result is no trips at all.
If you think there's a genuine affordability issue, tell school and have a discussion around it.