I think the place I got to charges ~14 for a haircut, I hand them a $20 they give me 6 single back, and I usually tip between $4 and $6. $4 if it's some random person who did an ok job, $6 if it's the person who usually cuts my hair (and usually does a good job).
That seems pretty reasonable. I basically assume that they are making almost nothing for an hourly wage - probably a few bucks an hour - and I figure they "should" be making $15-20 an hour, so my $5 tip is pretty representative of the 20-30 mins of their time I use up.
Edit: To clarify, this is a "barbershop" that has about 6 people cutting hair. I figure most of my $14 goes to their overhead and shop expenses. It's not like the owner is the only guy cutting hair, etc.