The important thing here, Mystery, is you don't choose what you remove until the resolve spell step AND you don't have to pay anything until the resolve spell step. You pay for the effect of removing the poison, not for Purify itself. The casting cost of purify is essentially 0.
Thus, if they move things around before the Resolve Spell step, you don't have to pay any mana at all. You're still out the spell and the action, but you don't ever lose mana unnecessarily. If they let you get to the point where you're spending mana, the Resolve Spell step, then they are too late to stop it from having the intended effect and you will remove things exactly as you planned.