So why don't they just offer prices (EDIT: starting prices) at $150 instead. Then the plane won't be half full but perhaps 90%. The price for those last 10% of seats can go up or down 1. depending on the demand for that flight and 2. how much time is left.
Because they want it both ways, sell the tickets they were going to sell anyays/regardless of price, and then again the two super underpriced tickets to people who wouldn't have paid if it weren't $50; tada, you've made an extra $100.