Does this counter / otherwise interact with itself?
Yes. If I discard one and you discard one, Silvers will produce $2 for both of us.
If that were the case, maybe it'd make more sense to go back to saying that the value of silver increases/decreases by $1 (but doesn't provide less than $0). The wording as it is currently appears to be assigning values rather than being modifications of the original value, so it's not clear that they neutralize each other the way you described.
I was responding to the old version. I really need to keep track of what I'm posting before I post it.
As written right now, the last card discarded would take precedence. Which means some precedent would have to be set for what order cards are discard when two players (including the active player) are discarding.
If you're going to have it affect other players, it has to be a duration. If it is a duration, you need something to signify when the effect ends. As it is right now, it doesn't affect your opponents at all because "this turn" is your turn, during which they do not play any cards at all. Treasure-Duration... peculiar.
It
doesn't need to be a duration. Rather, only one of the effects will matter: the one that pumps your Silvers will matter if you discard this on your turn, the one that lowers other player's Silvers will matter if you discard this on another player's turn.
I worded it that way because it's cleaner. I don't see it as being difficult to understand, but given that you understood the card the way you think I intended it, I may have to look at uglier wordings.