I always ask what the deal is.
For us, I'm a teacher at a rather fancy private international school, with dc attending as part of my package. Their friends' parents are VERY wealthy.
Invites have always been fully covered (& their kids get to go cheapskate desert camping with us if they fancy it!) but I definitely always check: 'oh that's so kind of you to invite dc, I'm not sure of our plans [if in doubt blame flakey xh who might be visiting etc]. Let me know what I should contribute!'
Invariably it's 'no Corvus, habibi, your dc is our guest!', but I'd always check/offer.
Last time was dd1 off island hopping round the Med. Club class flights & 5 star everything - I know dd1's BFF's mum quite well so did say I didn't feel comfortable just freeloading. She was very clear that it was a perk offered by one of her dh's business cronies so all included & dd1 was a) taking the place of one of her other teenagers who didn't fancy it, & b) company for her mate, so she was categorically not expected to chip in.
I sent dd1 off with a generous float for ice creams etc, & instructions to pay for dinner one night. She was told not to be so silly, she was a guest, put your money away you daft ha'porth & that was the end of it (I sent wine & chocs as a token after the holiday).
Equally, I've sent dc off on breaks with cousins in the uk where it's clearly understood that you contribute. Absolutely fine & I've appreciated the invite no less.
Ultimately I've come to the conclusion that if I trust someone to go away with one or more of my dc, I'd jolly well better be on 'discussing contribution' terms with them.