In a way, this game is actually cheaper than other collectible games because you can see the figure you're buying (unless you buy one of the "mystery" figures). That means you're paying $3.99 for a known item, so you can build a customized team without relying on the luck of whatever's in the box.
Of course, you can't get certain Pokémon without buying the mystery figures, but it's possible to build a strong team without them, so once you've spent about $30-50 on the starter set, three extra Pokémon, and a few reserves, you're probably good to go for plenty of 6 vs. 6 battles.
Contrast that to something like the Pokémon TCG, where you have to spend $60 just to build something slightly better than a starter deck. (Although that has never stopped me, hehe.)