I'm building an advanced flight search website that automatically takes advantage of certain "tricks" like hidden city ticketing as well as choosing the cheapest set of destinations and automatically constructing a schedule, mostly out of personal interest and curiosity. The main issue I seem to be having though is that I can't find any good APIs to search for fares.

I know of the main GDS's, but Sabre's API only seems to permit round-trip searches (which seems very strange?) and Amadeus didn't seem to be interested in working me (I guess because they wouldn't make much/any money off me). For the most part, I'm not interested in building a full OTA with booking, I just need the ability to search for fares like ITA Matrix does. Also, since I'm just a lone developer, I don't have the money to invest in a monthly subscription, but I'm perfectly fine with working off a test API with a limited data set until I'm closer to having a final product, at which point I'd be more willing to pay for a subscription.

Does anyone know of such a service, or how I should approach the GDS's, or what ITA Matrix runs off of?