FEODUM.
Feodum is the card you want open with chapel on a game without Colonies. Feodum is the card you want combo with Trader. And Feodum is the card you want if you can wisely trash it for coins.
Let me say that you want open Chapel/Feodum even if you dont want to go Feodums; due you want exterminate all the copper and replace them for something better, or even if you plan go Big Money. Remodel Feodum into Smithy, Wharf or Nobles: You gain extra buy power with the adition of those 3 silvers and the Smithy card! Filling your deck into Silver is a decent deal. With the Trader's aid, you can put 7 silvers on your deck. This is insane! Now you can go not only Feodum, but Gardens too! Also, a hand with 5 silvers allows you to buy a Province or Platinum. Not bad!
So, you can try to go Provs with Feodum + trash, or go Gardens with Feodum + Trader, or even go Feodum with something to trash, extra-buy cards, and Feodums. Even you could Transmute the Feodums in order to gain 3 Silvers and a Gold. With the appropiate trashers on the board, you can easily sustitute your Copper into Silver. Think about trash your Feodum with your Trading Post!
But, what if there are no Trashers?
Still there are few cards you can combo with Feodum. Trusty Steed gives you 4 Silvers! Also other cards give you Silvers, like Burocrat, and with Market you can go for double silver when hitting $6. This way is slower. Still, you can play Feodum as a extra VP when going for Provs instead the classical Duchie; you need only 9 Silver on deck.
Also, on a games without engine cards (+extra actions) and curse givers, you can go for Feodum/States instead Duchies, due on these games, hits $5 should be hardly. Only remind buy Silver when $3.