So far, it seems like a pretty cruel gacha game. 1 Magic Stone for killing 1 rare species. It should be 2. Maybe 3 if they're feeling generous.
Assuming that his first roll was heavily discounted, we can conclude that you'll need ~ ≥50 Magic Stones for a single summon. And even then, you won't be guaranteed something of high rarity.
Hopefully the story isn't 60% grinding.
And I wonder how the duplicate mechanic will be like...