Scrap field around bornholme depot in omega 15 is really high yield as well. Most scrap cans you shoot give around 10-20 scrap metal, which sells for 500 credits per unit at bornholme. I would suggest using a Junker ID if you go there though, I've been known to 'pirate' non junkers in that field =P
Silver in colorado isnt bad either, but you have to go all the way to manhattan to sell the ore for a decent price.
If you want to mine silver, try hiring new players to help you fill your hold, then fly it up to Kusari for sale, split the profits. Encountered someone else doing that, was good fun (as fun as shooting little rocks can be).