A couple of times i had suggested a recycling machine. I still think this would be a good way to actively generate some income, also would kind of produce some base line values for in game items.
It would have to be a very low value income, ie 10 rocks=1c, something like that.
And each time a base item is processed it would lose value, to avoid the refined rock xp machine, then recycle the ref rock for same amount of coin as base rock.
Using that for an example would give people a choice, do I grind up my rock for coin or process it for xp.