What are the best practices & technologies used by airline reservation systems, and are these accessible to developers in some type of pay-per-use model? Probably being naive but I figured if I understood the communication and systems involved I could probably do a better job than some travel agents I rely on.
What you're looking for is the global distribution systems (GDS)
There is a pretty huge industry around this and a few providers of the info you're looking for (inventory, pricing etc.). AFAIK they do use a pay-per-use model but it's typically a huge lump-sum to be setup then smaller charges as you query the system and create reservations.
These two wikipedia articles and their links should get you started:
The leader that pretty much everyone connects to is Sabre Travel. It has been years since I worked on travel sites, but you used to be able to make web service calls to retrieve airline, hotel, etc. pricing and even create reservations. The API is huge and in the case of hotels, much of the data is incomplete or not well formatted which is a constant pain to anyone trying to offer end users something better than what is already out there. Which leaves most sites offering their own content (hotels.com) or getting content from users (tripadvisor.com). Even so, once over the steep learning curve it is a usable API.
There is another large company in the US like Sabre whose name escapes me, but it is the same kind of setup.
An alternative would be to connect to a service like Kayak Search API to retrieve costs. I'm not sure it allows booking through the API though.
If you end up going beyond airline reservations, I have experimented with exceptional ones for random travel-related needs like vacation home rentals, yacht rentals and tourist activities. There are many web service based APIs available in these niche markets.