This seems too good for $4. It's almost always going to be at least a copper, most of the time at least a Silver. How often is it Gold? As often as Bank is worth $4, and that's reasonably often in the right style of deck. But at $4, it seems too easy to add these as an engine component; you can pick them up on the cheap to give you $5, $6, or so every time you play them. That's too good for $4, so I think $5 is fair.
Qvist, remember that logic breaks down above $6. Platinum is +$3 over gold, but gives an extra $2. Bank is in that fuzzy region.