1. plan of salvation is more powerfull than this card because of 2 circumstances:
1. It replaces your hand with wall+ cards, which is perfect for defensive decks
2. prosperity is conditional, meaning that you need less stock for your enemy, against fast offensive decks, this is quite impossible.
2. Currently, this is one worst rare recource gainer, because of its condition.
1.When you have more recources than your enemy, this card does even give less stock than Waelth (
http://arcomage.net/?location=Cards_details&card=129).
If the condition is fullfilled, you gain 36 recources, lets compare it with other conditional rare reource-cards:
-Ancient Sage: With fullilled contions, this card adds 15 stock at normal condition while draining 10 => -30 + (5 times # facilitys): better with 16 facilitys (which is quite easy to achieve if you play the right deck type)
But: Nature-Keyword, making it easy to be summoned
-darkforge: -27+(10 times #Dungeon) (6 Dungeon) garantued recources, with alliance keyword even more -27+(20 times #Dungeon) (3 Dungeon), this card is far superior than prosperity.
-Angel: +3 Stock for every holy? (4 Holy required) we need nothing to say, this one is far better
-Infinite weapon works: +20 recruits per round for the loss of your production...
This card would be the one you should want to have in fast decks instead of prosperity
-Clay Valley: +38
unconditional recource gain...
-Alastor flame of heaven: +25 recources + Banish +1 Magic, even for its high cost it is still better to consider than prosperity
-Magic Maelstrom: +2 Magic +29 gems for 1 Dungeon or Quarry, since this card has no drawback condition like prosperity and gems are needed in many decks, it is still better.
-Princess of Elves: A rare that gives 10 recources and summons another rare? Nothing to say here.
All in all i would say that Prosperity is only usefull in decks where you play fast, and even in rusher decks you would more like to have infinite weapon works or Darkforge. You would be even don't bother to give your enemy stock, so you would even then consider to use wealth, which is a common. I think this card even needs a buff when the condition is not fullfilled.