I don't know what's going on in this thread, but the idea of exclusion testing intrigues me. There are two ways to think about the strength of a card:
1. If one player has access to the card, and the other doesn't, how many more games do they win?
2. If all players have access to the card, how much does it impact the game?
For most cards, both kinds of strength are the same. However, Fool's Gold is an example that has more type 1 strength than type 2 strength. If you're the only person with access to the Fool's Gold, then you can always get 10 of them. However, if all players have access, they can foil each other, and it might not be very good for anyone.
You can also consider attacks that are good defenses against themselves. This is kind of a subtle point, but Young Witch sifts through cards, which mitigates the slogginess of having a cheap curser. Or Pirate Ship, which provides non-Treasure payoff in games where you may need it. These reduce the impact that a card has on the game, which is important if the impact would have been very negative. (Of course, the impact isn't always negative.)
For this reason, I think it's important to give a debt attack a payoff bonus (as opposed to, say, drawing cards).