Well, the problem any Silver related card has over Copper is that you already start with 7 Coppers, but 0 Silvers.
You need to buy Silvers from the Supply. So if there is ever going to be a Big Moneylender, it may take a while to set up a good combo with it.
If there are going to be other Big Money helpers, I think the cards have to do something for every Treasure, not just Silver, like this Moneylender-esque card:
"Interest"
Treasure
Cost: 6 (5?)
+2$
When you play this, trash a Treasure card from your hand, +$ equal to half its cost in coins, rounded down; or reveal a hand with no Treasure.
Copper gives just +2$ in total.
Silver gives +3$.
Gold gives +5$.
Platinum gives +6$.
Harem gives +5$.
Potion gives +4$.
Philosopher's Stone gives +3$.
Loan gives +3$.
Quarry gives +4$.
Talisman gives +4$.
Contraband gives +4$.
Royal Seal gives +4$.
Venture gives +4$.
Hoard gives +5$.
Bank gives +5$.
Diadem gives +2$.
Horn of Plenty gives +4$.
Stash gives +4$.
You can always play this safely as the last Treasure in your hand if you don't want to trash something and then it's just a Silver. It's a handy card for trashing those nifty early game Treasures like Quarry, Talisman, Loan, Potion and of course Copper. I moved it out of the Action phase to differentiate somewhat from Salvager. This also solves the problem of: "It's better than Moneylender, but not a very good action card, since it's worse than Salvager." Making it a Treasure can prove very useful.
However, playing Venture and having to play this with Platinum in hand can have unwanted effects, but this may be solved by playing Venture last. Also if you have this and Bank, you have to think about the order in which you want to play them. So there may be a few problems, but overall I think a card like this has potential.