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.
is that the official rule? thats my question
That would mean on dispel you also pay during resolve as it is the card text, but there you pay in pay cost as x is specified and purify has also x casting cost. I know the problem with that it is not clear.
I wanted to know what is the official end rule for it
My personal guideline is that Zuberi knows more about how the rules work than AW
The devious culprit here is "X".
From the rules:
Step 2: Pay Costs
You must pay all costs for the spell: reduce the mana in your Mana Supply equal to the casting cost (or reduce the mana on your Familiar or Spawnpoint by removing mana markers).
Some spells may have additional costs (such as taking damage, lowering your Life value, or destroying an object in play). These costs must also be paid now. As soon as you pay the costs required, your spell has been cast. If you cannot pay all of the costs, the spell is cancelled and discarded, and you have lost the action.
Lets change the example of the card in question to better show what is happening here; at least from where I am sitting.
[mwcard=MW1I11]Explode[/mwcard]
Step 1: Declare / target the mage
Step 2: Pay costs. The cost of the spell requires you to target an equipment on the other mage. You cannot " dead fire" a 6 mana cost explode on a mage with no equipment. The additional target line is not an option, but a requirement to cast the spell so you have to pay that cost during step 2, and as such has to be paid for in step 2 as per the bolded part; which does the most important thing, set the X variable equal to something. Without that there is no way to know how much to pay for a spell.
During step 2 you have to pay all the costs associated with that spell. If you are paying part of the cost for the spell after step 2 then something is not working right. The spell wont take effect until the resolution step but you have to declare something to resolve. The biggest issue with why Purify is a "problem" is that there is no "X = " in the card text. What that does is literally say everything tied to this card is an extension of the resolution ability and not an variable cost to cast the spell. and as such it plays out as Zuberi described. That means in the specific case of purify, or anything where X is not specifically given a VALUE OR OTHER CONDITIONAL COST, there is no cost until step 4.
