Where coins can be found
Golden Coins can be found in any object that is considered an NPC - This includes townspeople and even doors. If one has the Gold Hunter Perk, farming coins becomes much easier. Golden Coins held by merchents can be bought in the store for a fairly cheap sum of Silver, otherwise they have to be killed to collect it. Doors holding Golden Coins must be destroyed via ranged attacks, or else the door will "open" (disappear) and the coin can no longer be collected. The higher level the campaign, the more often Golden Coins can be found on NPCs. They can also be found laying on the ground, with the chance increasing as you go farther in distance.
There is also a Golden Coin bonus award at the end of campaigns after winning the game. The amount varies depending on the difficulty.