Well, I did note that it might be interesting that way too.
I think a full -$1 penalty would be bad and unfun, especially when stacked. I would suggest this:
Inflation$4 - Action-Duration
+1 Card; +1 Action
Each other player takes a coin token.
Now and at the start of your next turn: take a coin token.
While this is in play, when a player spends a coin token, he returns an extra coin token to the Supply.
If it is not a mirror, then your opponent gets a bonus like with Council Room, except it's essentially +$1 instead of +1 card. If they only got one extra token, they could spend it freely now without penalty. With multiple tokens, it's a more interesting choice.
For you, you could spend your single coin tokens immediately to avoid the penalty as well, unless you play multiple Inflations on your turn.
With multiple Inflations getting played per round (whether in a mirror or multiples from one player), it becomes that weird situation where everybody is gaining more and more tokens but they aren't worth much more.