I think this card needs a little something extra for when there's no trashing available.
And because of that I thought the card needs the trash function for itself (similar to graverobber and many other cards). I wouldn't never buy that card for 5 if there is no trasher available and if there's a trasher then the filter-bonus is of little use anyway. But that's not too bad.
Right, that was my thinking. The filter bonus is more useful when there isn't trashing.
But isn't a start with chapel not a bit too strong and also swingy? What about Forge or Count? I really would limit it to max +$1 or +$2 per trashing.
That's what I thought, too, but after consideration I don't think it's that bad when compared to what's already out there. Consider a Chapel/Market Square opening. That can get you a Gold on turn 3 or 4 if they collide. If Broker and Chapel collide, you get a $5 of your choice. That's often going to be better than a Gold, but not by much. Sometimes it'll be worse. There is the argument that you actually need a 5/2 split to open Broker/Chapel, whereas anyone can open Chapel/Market Square. Also, if Chapel and Broker don't collide, it's a heck of a lot better than if Chapel and Market Square don't collide. Still, I don't think this is any more swingy than a lot of other 5/2 openings, like Mountebank/Chapel or Mint/Fool's Gold.
As for Count, you'd have to buy Broker and Count, two $5 cards, then have them collide together in a hand with no other cards you want to keep. Sure, you could put a card back on your deck, but that means you'll only trash 2 cards and will only be able to buy a $4 card after trashing.
With Forge, you're not going to trash a massive number of cards with it more than once or twice per game. If you can draw a Broker or two, and Forge, and a bunch of cards to trash, and some +Buy (to spend all the Broker money) into your hand at once, you deserve to buy some Provinces.
I'm going to playtest this some more soon and it could be that you're 100% right about it being too powerful a combo. I'm hoping that the fact that Broker doesn't give +Cards (limiting the number of cards you can mass-trash), and the fact that there's a lot of competition at the $5 level will make it a good, but reasonable option.
EDIT: If it does turn out to be too powerful, I will look for a way to limit it somehow. The 'when you trash cards with an Action card' wording seems a little fuzzy to me, though. 'When you trash cards' is technically well-defined, but not intuitive. 'With an Action card' isn't necessarily clear. I'll try to brainstorm something with a similar effect.
