If +1 buy, card cost 2$ less is Price-level power, than it seems quite unlikely that your card is balanced.
unpack this for me. I removed the buy because the price reduction combined with the +buy was too much; price reduction alone just makes it more useful in +buy games (same deal as quarry)
Princess is a Price card that gives +1 buy and makes all cards 2$ cheaper. Compared to Princess, your card has three big advantages
- Doesn't cost an Action (and can't even be drawn dead)
- Does stuff in addition to making things 2$ cheaper
- Can be massed
And two big disadvantages
- hard to use the cost reduction in your action phase since it's a Treasure card
- no +buy
Overall, this seems stronger than Princess, which is probably not good given that Princess is a price card.
The thing that makes Quarry work is that it only applies to Action cards where cost reduction has diminishing returns. If you play two quarries, you can pretty much get one action card per buy you have, but that's one step away from the payload. You're setting up your qurry thing just so that you can build an even stronger engine. Which is definitely good enough on some boards, but it's different from going directly to the green cards.
Perhaps look at it like this. Bridge essentially introduces an alternate win condition: "play 7 of me in one turn". However, this curve slopes steeply upward: playing 5 of 7 does not give you 5/7th of the benefit, rather it probably just isn't good enough (if you start greening then, you won't connect bridges in the future and probably lose). Your card also introduces an alternate win condition: "play four of these and about 5 cards with +buy". This is much easier to do if there is a cantrip with +buy because it (a) has fewer components that don't draw and (b) has no terminal components. Just market square + your card + trashing wins the game. And it also has a third benefit, which is that the curve is linear with regard to the +buys. Playing just 4 cards with +buy is, in fact, 4/5th as good as playing 5 (as long as you have 4 Royal Seals in play). You still get 5 province.
And because you don't need terminals, it's also doable without a +buy cantrip. It's not too hard to play two squires for +4 buys, or if your card drawer has +buy,that also works.
So this suggests to me that the card is significantly more powerful than bridge, which is probably not ok. Unlike with other cards, the fact that it costs 5$ and not 4$ doesn't change much since it's not about getting it early, and once you have a few, you're bottle-necked by +buys anyway.