Keeping up with the meta on a budget

ichooseYOU456

After casually collecting for a long time, I'm trying to get into competitive play, but I've always hesitated because of the insane speed at which new cards and strategies are introduced, before fading into obscurity just as quickly. I assume this requires buying new cards regularly, but I don't have the budget to just buy new shiny cardboard all the time. And when cards are no longer useful, they aren't good for anything but taking up residence in a binder, so it's not beneficial to buy beat-up cards for cheap. How can I keep up with Leagues and tournaments without breaking the bank, and have cards that are good enough for keeping after they've run their course?