My DH has three other children aged 16, 13 and 10. He pays above what CMS have stated and does all the travel to collect them (90 miles round trip each way).
He also pays half of everything on top. School trips, hobbies, prom, transport to school.
We have a DS of our own and I'd say I pay 80% of costs associated with him. I earn more than DH so I don't mind paying more however there is of course some contusion with all these extras he pays.
I'm curious to know what exactly CM should cover and what other people do.