Unfortunately, airlines have never taken Economics 101. Economic theroy would suggest that if the plane is taking off without being full then if there happens to be anyone standing around the gate that wants to go where that plane is going you should sell the ticket for whatever they are willing to pay. The extra person will probably not effect fuel costs and the only incremental cost might be a coke and a bag of peanuts. So anything they are willing to pay over the cost of the coke and peanuts you should take and let them get on the plane. Reality is if they did this sort of thing hundreds of people would hang out at the airport looking for cheap flights to Hawaii, so they charge more to discourage people from trying it.