From the Prosperity rulebook:
Bank card description wrote:
If you play Bank with no other Treasures in play, it is worth $1. If you play two copies of Bank in a row, the one you play second will be worth $1 more than the first one.Also:
Additional Rules for Prosperity section wrote:
A player may play his Treasure cards in any order and may choose not to play some (or even all) of the Treasure cards he has in his hand. During the Buy phase, a player must play all of the Treasures he wishes to play before he buys any cards, even if he has +Buys; he cannot play more Treasures after Buying a card.