Just to add a little additional info...Non-revs are usually broken into 2 categories, NRSA(Non-Revenue Space Avaliable) and NRPS(Non-Revenue Positive Space) 'Space Available' is pretty self explanatory, 'Positive-Space' is hard to come by but is generally applies to an extra fee that allows you to purchase a confirmed seat. ATA used to allow non-revs to purchase positive space travel to Hawaii from LAX
for $250 round trip and you could take a non-airline friend with you. Positive Space pass riders can usually be bumped if the flight begins to oversell. As a crewmember when I deadhead on line to position for a trip, I am booked as a Non Revenue Must Ride in these cases in the event of an oversale, a revenue passenger would have to be bumped to ensure that I had a seat on the flight.
As some have mentioned non-rev passes can be a flat fee or in many cases you pay a percentage of what paying customer would pay. Most non-rev tickets are issued at an ID90, ID75, or ID50 rate. ID
stands for interline discount and the '90' represents the percentage of discount. This is a 90 percent discount off the full coach fare. A ticket that would cost a paying customer $1000 can be purchased by an airline employee at an ID90 discount price of $100. Tickets can somtimes be purchased at the ticket counter on the day of travel but in most cases you must go through your company's pass bureau which verifies your eligibility for the discount(some carriers make you work for 1 full year before they extend a discount) calculates the fare and issues the tickets. The cost of passes in many cases can be deducted from your paycheck.
With rock bottom internet fares available on many routes, sometimes you 'discount' is not the cheapest way to go and spending a few extra bucks is worth knowing you have a confirmed seat.