Text (LG+errata): Whenever a player casts a spell, counter it unless its controller pays {3}. [Oracle 2009/07/08]
This is not an additional play cost, it is a triggered ability. [Duelist Magazine #9, Page 60]
The countering effect does not take place until after Nether Void resolves and is in play. It does not affect its own casting or any casting in the same spell stack. [D'Angelo 1995/12/09]
Extended tournaments (see Rule 913) have always banned this card.
