We were the inviters. It was all for our benefit - DD is an only child and generally had a nicer time on holiday with a friend along. Always in the UK, so no flights. Accommodation - as a family of three you usually end up with accommodation that sleeps at least four anyway, so no extra cost. A bit more for food, obviously. Sometimes more for activities but not always - having an extra child sometimes meant they did a thing together rather than one or both adults needing to join in.
It was almost always the same child, and her parents would send cash in an envelope, and we’d send it back with her. Thinking about it now, we probably should have used a bit to pay for a meal or something, but it never seemed necessary.
I don’t think I’d ever have extended an invite without intending to pay for all of it. The way I saw it, we only have one DC to pay for throughout the year, one extra for a week or two in the summer isn’t going to break the bank.
The only time it was reversed was actually a trip to CenterParcs OP! We sent DD with spends. I suspect it cost the parents less than if she’d not gone - their DD was the only one left at home by that point so effectively an only. When we went to CP we always spent loads on activities, whereas for that trip they mostly just hung out together.