It varies by route or cost of the ticket. Around USD1 to 5. Perhaps around 10% of ticket value or more. I used it quite a lot to request tickets before it opens at 2 weeks before the travel date, and at that time it is booked for me. Very handy as I was travelling with friends. Don't need to be afraid of missing out.

The official site seemed painful to set up but I eventually did it. Will be using it for the first time in two days.