3x your total out of pocket expense + what you feel your time is worth. Be it construction (my day job) or BBQ, I don't wake up for less than $30 and hour. I also don't let the customer (weather it is a friend, or just a client on a professional level) supply the meat due to quality and food safety reasons. If it is for a friend, charity organization, church, something like that, I would be more than happy to go to the store with them, pick out the ribs and take possession of them.
Dedicated Stick Burner, Pitmaster @ All About The Q, facebook.com/itsallabouttheq KCBS CBJ