How much does it cost to tune a piano?

Piano tuning costs vary greatly across the nation, even across the local markets - much depends upon what you are getting - are you getting a full service from a competent technician or, maybe not - but that's another question for another day.  Regardless, never hire anyone, a piano technician or plumber, etc., based upon price.  Do a little research on the person your hiring - get referrals, check out their website, call them and ask questions!
As for an answer to the question, right now, prices seem to range nationwide from $85 to $150 for basic tuning service.

To learn more about my fees and what you get for your money, go to http://www.freestatepianoworks.com/fees.html.