You can even sign up for free... try out the base level cards and see if you like it. If you decide to join and to buy cards and level up ... then you need to buy the summoner's spellbook. But that also lets you win reward cards that can be sold for as much as $50(Gold foil legendary) if you are lucky enough to flip that special card.
So, yeah.... it is fun and exciting too!! Give it a try!