I can't really remember how much on my main one as I have been playing on it for quite a long time now. But I do remember how much on my alt accounts.

I only invested around $30 USD I think on it. Bought a level 3 Beta Earth Summoner, 1 Beta Water Summoner, leveled my Flesh Golem to level 3 to get that Heal Ability, Bought that Earth Fairy that gives tank heal, bought 1 Ruler of the Seas and from there I was able to grow that account to 100k Collection Power now xD

Those cards are quite expensive now though and took me around a year to grow it that much xD