Although my DC are adults, I usually cover a lot of the costs if I go away with either of them. Partly because they earn a fair bit less than me, and also I couldn't afford to take them away much as kids due to being a single parent, low wage etc. I'm happy to do that.
There's an event next year that myself and DD have been to once before. We talked about going next year and a friend of hers coming along (I've met her friend a few times and she's lovely so no issue there).
However, due to the popularity of the event, the cheapest accommodation I've found is £1700 for the week. So I've said to DD that if we are going, I will pay £700 but her and her friend will need to pay £500 each. I'm prepared to pay a bit more as it's a 2 bed place and it's likely I'll get a room alone and they'll share (apparently I snore!). Her friend does work but again not highly paid, however they do have several months to save up between now and the event. Does this sound fair?