There are two different ways to connect to an airline inventory system (what you call the catalog). Either direct, by signing a contract with the airline, and building the necessary "pipe" (IATA are pushing for a standardisation of this, called NDC). Or through a global distribution system (GDS) - which is a fancy name for what used to be called computer reservation systems. There are three major systems, by decreasing importance: Amadeus, Sabre and Travleport. They will cache and distribute the inventory and fares for all client airlines, and also allow you to perform bookings. Every travel agency in the world (including online travel agencies like expedia) is connected to one or more of these GDS.
Now the pricing can be complex, a good first order approximation is that you pay a fee for access to the GDS, that varies depending on how many queries you are doing. For a traditional travel agency this fee will be very low, obviously for a major OLTA it will be big. However, for every booking that you perform, the GDS will give back a sizable amount of the distribution fee that it took (we are talking several euros per flight here). So as long as your "look to book" ratio is reasonable, the access costs will be small compared to the incentives you make.
Edit: Why are airlines pushing for the "direct" channels? Because it means they can cut out the middleman of course, the GDS. The problem is that this makes it hard for a travel agency to connect to every airline - the fixed costs of setting up pipes to every airline would be astronomical. And there will be an incentive for agencies to negotiate preferential deals with certain airlines. Both of these factors are obviously negative for the end user, travelers. It's a similar question to the whole net neutrality thing