You can easily determine what the impact of a price increase will have on load factors based on past fare increases and their affect. They teach it in any MBA program as a way to figure optimum pricing. My guess is the price sensitivity is strong, which is why they don't raise fares but add on luggage charges, etc., that don't show up on the www.cheaptickets.com search.

Two things to consider: One is belly cargo; the second is that fuel really isn't a variable cost - how much does the fuel load change for 1-2 people on a flight? Not much...so you're paying the majority of the fuel costs simply by flying the plane whether it's full or not. This is the reasoning behind the super-cheap fares 1-2 days before a flight with open seats - they want to recover some of the costs, not generate a profit.

HTH
Spongebob