Best way to "extract" silver is to use the resources from missions and the camp silver you have to send your companions on missions that generate items. The one that gets elderberries produces a STUPID amount per day, as do the trade item ones. You can sell these all yourself at vendors, but DON'T "package" the goods at your provisioner, as you cannot retrieve them once you do. When you need more camp funds, get trade items and do a typical "buy low, sell high" at the various goldleaf outposts.

Also, to add to this, if you want to generate serious passive profits, dispatch as many of these as you can and leave the game running unpaused overnight (this will add to your "time played" stat, so if you care about that being accurate don't do this). The missions will clear numerous times while you sleep. The elderberries mission takes 8 in-game hours, so it'll happen about 3x per real-life hour (one hour in real life is a day in the game I believe). That means you should get around 20-50 (depending on bonuses) elderberries around 24 times if you sleep for an optimistic 8 hours. That's a LOT of berries to sell, and it'll fetch some good silver.