I've never charged a friend or family member for building a PC. Once or twice I've taken the opportunity to sell them my old gfx card or RAM so I can upgrade, but always made sure they paid a fair price. On other occasions, I've even thrown in an old HDD, cheap gfx card or spare stick of RAM I had lying around for free.
It would feel wrong charging a mate for an hour or so of fairly easy work. I like to put the stuff together while they're there anyway, and chat about stuff whilst doing it, so it's never a chore in my experience.
If it wasn't a friend, though, or a friend asked me to build for a friend of theirs, I'd probably ask for £25 or so since it would probably take around an hour.