It is certainly possible for it to break the normal rules and defer the casting cost. You are correct. However:
A. It does not clearly communicate that such is what it is doing.
B. It would be a lot simpler to just write the casting cost as 0 and then write that the destruction effects require extra mana to be paid at the time of the destruction, like it is currently written, than it is to explain that the casting cost is paid at a different time than normal.
C. Inconsistent use of variable casting costs can cause confusion. Which spells pay this variable during the Pay Costs step and which ones pay it during the Resolve Spell Step? A confused soul could easily assume that spells like [mwcard=MW1I07]Dissolve[/mwcard] also work like Purify, and thus you don't lose any mana if it gets Nullified.
I'm okay with it breaking the normal rules and behaving how Laddinfance stated, but it is horribly written currently and needs errata to clarify how it works.
@Exid:[mwcard=MW1E29]Nullify[/mwcard] always costs 2 to cast and 2 to reveal regardless of what it counters. With the transfusion, previously I would have stated that you can't reveal it during the Resolve Spell Step, but with the discussion currently going on
here I am unable to say that with certainty. You might be able to reveal it after they've removed things with Purify.