Herb farming is the best by far, although they recently nerfed it so that you have to sell the tinctures to get the best profit instead of selling the seeds.
Be sure to find the Tool Vendor and check those prices, too. With upgrade cards, you can make some good returns on Tower bargains and resale to the Tool guy.
Humbabella, as is his wont, crunched the numbers, and came to the conclusion that mining is not an especially efficient way to make money. Most people prefer herb gardens these days, or you can try to grind a feat (whenever the next one comes) and hope for an expensive artifact to sell for your trouble.