ID ticket prices
Many airlines subscribe to what is known as the ZED fares agreement for interline staff travel. Ticket prices are fixed according to distance bands, and then can be 'low' 'medium' or 'high'. For example Western Europe to East Coast fall into a band that ranges from a low of $59 to a high of $136 for a one-way. However certain taxes and fees are usually added on top which in Europe can sometimes be higher than the actual fare itself.
Some airlines use ZED fares to charge their own employees, while others have their own internal arrangements. There is no hard and fast single benchmark.
And, no, I have never heard to an airline donating employee fares to charity.