I once flew from San Francisco, CA to San Salvador, El Salvador for an interview with TACA. I was beyond excited at the possibility of flying an Airbus. Turns out they wanted to interview me for an engineer position in their Aeroman aircraft maintenance facility. It was a $900 round trip ticket. The worst part is that they have a direct flight from SFO to San Salvador on which, even after I asked, they did not offer to get me on. This is obviously an extremely crappy situation, but things like this do happen.
My other concern is that if they are not paying anything to interview you, the selection process may not be as rigorous. They will probably invite ten times more people than what they really need. You will make your way across the world just to find out you just "sort of" fit the profile.
rcl