Text (TE+errata): Creatures can't attack you unless their controller pays {2} for each creature he or she controls that's attacking you. [Oracle 2007/02/01]
The payment is made during the declare attackers step at the same time you are declaring the attacker. [WotC Rules Team 1998/03/01]
Paying Propaganda is not an instant or any other kind of ability, it is a cost increase on the declaration of the attacker. This makes it much like the way Gloom makes a white spell more expensive to cast. [WotC Rules Team 1998/03/01]
If there are multiple attacks in a turn (due to Relentless Assault), then you have to pay for each attack. [D'Angelo 1999/06/01]
If there is more than one Propaganda in play, the cost is cumulative. [D'Angelo 1998/03/16]
