First time, if all the parts are provided, I'll help for free or trade for a few beers (if the place where this is done allows). I consider that a mini-teaching course / chat.
If the person is badly stuck with a horribly messed-up job, I may also do it for free Once. Same if they are at a game and screwed.
If the error is repeated due to stupidity, I would charge more. Along the lines that Illusion quoted.
A ballpark figure? $20.