Well if you're into the game, yeah, you'll probably want to buy new cards once in a while when they come out with new stuff. But unless you're playing in tournaments, you can use any cards in your collection in casual games, and some tournaments and events use the much more lax 'modern legal' rules, which allows you to use any recent cards from a certain set onward. It's really not all that expensive to have as a hobby, and it's a lot of fun.