Wait, first thing: did the school mention if you could pay in instalments? Because honestly, that's often the step people forget to ask about directly, and it makes a huge difference for the budget.
On the financial aid side, the previous answers talk about where to look, but you should also check with your local town hall or the département council - sometimes they have grants or funds for school trips that nobody knows about. Some regions have specific programmes for classes vertes. And honestly, talk to the school about your situation: headteachers see families in the same boat almost every year, and they can sometimes find discreet solutions, like discounts or local partnerships.
If it's really a blocker, parents also club together through sales (bake sales, second-hand markets, etc) at school to fund this kind of trip. It's not super glamorous but it works. The main thing is not to wait until the last minute - the sooner you tell the school you're struggling, the more they can help you out.