back when I had my firefly transport (3495 cargo) it took around half an hour to get a full cargo hold, roughly - and a full hold would make under 2 million credits (1.9 or somewhere around there). It's not even close to powertrading, and it takes a lot more 'paying attention' to mine than it does to set a waypoint and hit F3 a few times.
That field is critical to the faction I'm trying to run - we're supposed to be the 'legal' junkers, focusing entirely on salvaging and mining operations over smuggling and piracy. If our field gets taken away, we're shot back to smuggling again.