You heard that right. It's 1$.
It's a trading card game that uses preset 'formations' instead of decks, so there's no real random factor except for certain monster abilities. Either way, you set up a formation of up to 6 monsters, half of which can be 'hidden' - completely invisible until the monster they're backing up dies, letting you set up traps. You'll have one monster 'summoned' at a time, but the others can back them up by paying Mana to activate SP skills; and monsters often have multiple attacks to use when summoned. Despite the fact that everything is monster-card-based in practice, the strategy options are surprisingly huge - what with having something like 235 cards (Doubled, since you can turn multiples of cards into a 'premium' card). It also comes out surprisingly balanced - with an Elemental RockPaperScissors system present, and cards having different costs that lower your initial mana and prevent you from over-stacking on high value cards.
Seriously, it's awesome. And did I mention it's online enabled and has a pretty massive single-player? The only real downside is that single player only gives you access to two 'sets' of cards throughout, apparently; the rest are only recieved, win-or-lose, in ranked matches. And those are limited - you'll have to use 'tickets' to get in. Of course, the game starts you with 10, and you can win more in Tournaments (which are free to enter, and the top 85% every tournament win 'em), so it's not that bad. You could also buy the packs directly...
But you don't have to is the thing. This is a game that, with time and effort, you can actually get every piece of content in-game without ever paying more than the 1$ entry fee.
HAVE I MENTIONED IT ONLY COSTS YOU A BUCK? BECAUSE IT DOES. AND IT'S HUGE. GO GET IT.