(Again, I found that it was $30 for 20 champions and extra in-game money.)
However, all you really need to play are just one character of each 'class': support, DPS, caster, and tank. Of course, knowledge and access with other characters may be beneficial, you just need to know your main specialties.
Heh, the tutorial mislead me to believe that the game was made for the younger audience (I can assume the art style is supposed to be more unique and anime-like than child-y, if that sounds paradoxical).