I just checked out where I took lessons for awhile....$25.00 per half/$38.00 for 45 in/$50.00 per hour. And that is in Denver proper. If you are a member of the folk music society they give you a $2.00 discount.
I'll just pay the $2.
Everyone around here charges from $10 a lesson to $100 a lesson. It really depends on what you can. You get what you pay for (charge what you should). Also, keep in mind that professionalism (on time, prepared, having policies, etc), being personable (not a weirdo, this one in particular can put you above the rest), and knowledge (experience as a teacher and a player, ability to answer questions about theory, technique, etc) are all factors on top of your skill as a player.
Oh yeah, I found that, in my area, you don't have to "give a free lesson to start" or something. It give the impression that you or the teacher unconfident (no offense to people that do, just how it works here maybe?). Also, it takes more than one lesson to know if it is going to work between a student or not. I have mine pay a month at a time, including the first day: "Four weeks please!" By the fourth week, they understand what I am talking about a little more and usually don't stop taking lessons after.
We have a massive amount of folks who are not qualified to teach music in the area who advertise "I'll come to your house for $20 an hour" that end up taking a lot of potential new customers off the playing field who don't realize that there is a difference in quality between the dude who has no education and will just show you what he knows by wrote vs. a highly educated and experienced musician who also happens to be a good teacher.