I wouldn't be surprised to see a strong secondary market creep up in this game. I already found one seller. Give it some time, and I think you'll eventually be able to grab anything you're looking for without having to shell out for entire expansions. That said, yeah, this game is pretty expensive, but it's ok, too, because if you teach someone to play, and then explain that if he gets a core set you will both be able to customize your decks more, I think it's easier to get people to buy the game rather than just mooching off your card pool (which happens with a lot of games I play...generally one person in my group will buy a game and then we all play that person's copy...I think this is probably true of a lot of groups).
-nihil