Back in my NR days (I worked in technology), it was something like $10 per one-way trip for Y, $35 per one-way trip for F. We also received up to four annual no-cost "vacation passes" which had slightly higher priority.

At no time (on my carrier) did any employee ever take a seat from a paying customer - we were seated only when all eligible, paying customers had been provided a seat assignment (I realize that some carriers differ in this, notably UA).

In addition to this, there were opportunities to "interline", that is, to purchase deeply discounted standby tickets (20%, 50%, 75% or 90% off of full Y) on certain other carriers. Typically, when using one of these "ID" discounts, you travelled at the lowest possible standby priority ranking.