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Do you think that load factors in the high 70's low 80's is enough? 
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Yep. If you PLAN for load factors in the mid to high 80s and set your prices to reflect that, you're gonna do well during tourist season, but unless you raise the prices to make up for the drops in load factor in the off season, you're going under. For example, SWA generally makes a decent profit off an 80% load factor on a 737-700 during the summer, even with a lot of the discount tickets. Keep in mind that whole 80% is NOT a discount fare. Maybe 30% of the seats are offered at that rate, and once the seats are sold, they're gone for that flight. Anyone else has to go with the next tier price and the restrictions (or lack there-of) offered on those tickets. Since that bottom 30% was probably non-refundable and non-transferable, the airline doesn't really care if the people show for the flight or not once they have the money. Now, if you sold the WHOLE airplane for $29 each way, then you'd probably have to run about a 90% load factor to break even.
You also mentioned freight and mail, which are HUGE money makers for airlines, especially SWA. We used to have management breathing down our necks to make sure the mail and freight got on the flights they were supposed to. My last year as a ramper, SWA even went with $100,000+ mail tracking system. New rules for mail contracts said that anyone with a US Postal Service contract needed a tracking system. The boys in DAL figured the money we would make from the mail contract far outweighed the costs of implementing a tracking system, so it was done. Too bad scanning a bar code is too difficult for some of the rampers, so a lot of that mail flies for free  
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 Admittingly, I've read a few books on airline economics but I doubt I will ever understand their pricing structures. $380 to go from MHT to CLT on USAir but $149 to MHT-FLL is farther and costs less? 
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Depends on when you buy, how many seats are left, etc. If you buy earlier, you stand a better chance of getting a better deal since some of those low price seats (with the restrictions) are still available. The loads on a MHT-FLL are gonna be higher than a MHT-CLT since FLL is a tourist destination, so the price can be offered lower in order to attract volume. If the load factor is less, you have to charge more per ticket in order to break even. For example, say the break even point for the flight is $100,000 (WAY over simplified, BTW). You figure on the MHT-CLT run, you're going to have a 40% load factor. Using SWA's 737-700 seat config (b/c that's really the only one I know), that's about 55 seats. So, you need to come up with $100,000 using those 55 seats. Anything above that is gravy. You can mix and match the seats by offering 30 seats at the fun fare price (non-transferable, non-refundable, only on wed, sat, tues, etc), 25 seats at the next tier price, 25 at the next tier price and the rest at full fare with no restrictions. You're hoping that some people will buy the full fare ticket price to avoid the restrictions, which happens more times than people realize, especially with frequent fliers (I didn't realize this until I worked ticket counter at XJT and saw how many people actually DO pay full fare). Now, on the MHT-FLL run, you're looking at a load factor of around 80% since it's a tourist destination. You can offer MORE of the fun fare and other lower prices since you know you'll get more people on the flight. The airline knows they can get more people on the plane due to the destination, so they more or less sell the seats at a bulk price rather than individual pricing, sorta like shopping at Sam's instead of Target.