Have a look at ExpertFlyer, their premium subscription plan offers the following features:
- Airfare prices are available in the currency of your choice
- Shows all published airfare prices between two cities
- Airfare prices displayed for one-way, round-trip and Around The World (RTW) fares
- Shows both booking class and fare basis code
- Specify a particular booking code fare or see all fares
- Choose up to three particular airlines or see all airline fares
- View detailed fare rules and routing rules for a particular airfare
- Search for airfare prices based on advance purchase and seasonality restrictions
There may be other services offering the same functionality, this is just the first one I found.
However, services like these are essentially just front ends for the GDS [Global Distribution System]. There are 4 main players that make reservation systems: Amadeus, Galileo, Sabre, and Worldspan. Most airlines have a reservation system provided by one of these 4 companies, the only exceptions are low cost airlines.
Depending on your needs, contact each of the GDS systems and enquire about getting access, they will all provide data access, although at a cost.