Don't forget commander, which a lot of places claim is now the most popular format. Pre-constructed commander decks can cost as little as $20-40 and competitive commander decks can easily go into the thousands.

The game also has a very high skill ceiling. I think that's one of the main reasons why magic has such a broad age range to its player base. There's plenty of weird lines of play, from strange card / rule interactions to weird deck themes no one else would think of.