MY BIL travels with us for many of our trips. We are me, DH and DD (10). We usually split 50/50. Here's my reasoning:
1) Without him, we could probably squeeze into an economy or small car - whereas, with him we've found we need a full size,
2) If he didn't ride with us, he'd have to rent his own - or use a shuttle. Of course now the shuttles (ME ) are free so I don't know if he would opt for that.
3) Most of the time, I get pretty good prices on rental cars - This October, $108 for full size for a week - all taxes included.
BTW - He also pays 50% of room cost, which I base on beds. DH and I get 1, BIL gets the other. DD is put on an air mattress on the floor (usually much to her disappointment) LOL!