I always believed that last minute tickets were the most expensive. However, when I missed a flight from EWR-OSL last winter I was able to get a $500 AI ticket on Icelandair 3 hours before departure. Granted, it was low season on Icelandair, it was internet/phone not walk-up, and I got stuck in a middle seat in Y but it was indeed cheap. And Icelandair doesn't always offer the lowest advance-purchase fares.