Looking for this game...why is it so expensive?

Just recently platinumed Killzone 1 on ps3 and thought id hunt down the ps3 version of the game thinking it should be easy to find. Turns out its not as easy to find as I thought and it is usually around 100 - 120.

Does anyone have any idea as to why its this high? Is it because of region based or just limited print? I do know that this came in the Killzone Trilogy set but thats about all I know about it for now.

Any knowledge will help.