There's no conservation of mass in a world of magic. Of course in RL it's not strictly speaking a law either, since the real thing is conservation of both mass and energy, with mass being interchangeable with energy. Nevertheless, the Ludia core is cracked, but to regenerate, they will need 20kg of new matter. We don't actually know the density, so we don't know how big lumps 10kg of each of that stuff are, but if we only consider the core, even if it's cracked, the material it's made of should still be there, right? Unless some of it evaporated or was converted into energy through a nuclear reaction. Or magical mass to energy reaction. So, the cracks would be gaps from where matter disappeared, not the effect of the core shattering, like one would normally think. In that case the 30 billion worth of stuff would fill those gaps. But if the core actually shattered, then it would be 20kg heavier after getting fixed.