Somehow, no matter how much I think about a card, I always forget the multiplayer scaling problem. I'm editing the original card and comments to reflect what I think is a more balanced version.
As for its weakness, I'm starting to want to move this up to 4 anyway. I really don't want this card being spammed as much as a good 3 could be. This is another option I'm heavily considering:
Brokerage #2 (4)
Action - Duration
This turn and during your next turn, all cards (including cards in players’ hands) cost $1 less, but not less than $0.
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While this is in play, each player (including you) gets +1 Buy during their turn and must use at least 2 of their buys during that turn.
It feels a bit close to Upgrade as far as stacking goes, but a bit easier to stack, and with multiple buys, so I wonder if that's just too good. I might just need to add more regular bonuses instead (+1 Card?). Also, as I think of this more, I wonder whether or not I should have the mandatory buy on one's self.