I charge $10/minute for customs, which I think is pretty standard. I used to only do 10-minute minimums but it's kind of nice being able to do shorter ones really quick so now I'm not too picky. I charge extra for name usage, lengthy scripts, new props/costumes, supply usage (like baby oil), or specific fetishes that I don't normally cater to or that I don't much like doing. I also charge extra if they want to pay a non-legit way since there's extra risk involved (I wouldn't take payment non-legit ways until you've been doing this for a while and know how to watch out for scammers and what not, though, imo).
I experimented a while back with a lower price ($6/minute) but I still seemed to get the same amount of customs - I even had a regular at the time who was buying customs for $6/min who didn't even bat an eyelash when I raised my prices again to $10/min. Guys that buy customs usually seem to know what price range to expect I think. It's only ever been the freeloaders who seem at all shocked by how much customs are.