Hello

I just ave a question about the second part of the card  ( Then, as a free action, you may immediately cast the destroyed conjusation, or a non-Terrain conjuation spell from your spellbook, tarfeting this zone. )

Do i need to pay a second time the mana cost if i keep the enemy conjuration?? do i need too spend casting cost a the conjuration i chose from my spellbook??

if someone know the anser le me know pls