With my parents, we foot the bill aside from travel and park tickets (they have Florida residency, do they get some nice tickets!). With one set of friends, it's a rough estimate of splitting. They know we pay X in dues for the points we used, and split that amount, but pay us by buying dinner and drinks or stocking the kitchen. They buy their own travel and tickets too.
We have one set of friends we bring who I don't take money from at all; they buy their own travel and tickets, but I don't take money for the room. We also often buy nicer meals for them, but they choose to repay in non-monetary ways (I don't demand this, it's our unspoken arrangement, I guess, we've been friends for 40 years)---babysitting for us a few times on vacation and do some cooking and grocery shopping.
I guess I based this more on who we invite than on my own costs.