Last run was in my cobra with a 3-bin refinery and 18 cargo space. Filled it up and made just shy of 100k. Best system is in Coelrind in the B ring you can then sell the stuff you mine in the same system at Siegal Hanger.
Nice and safe with no real piracy.
Secret is to be picky about the asteroids you mine. I only bother with ones that give good chunks 15%+ other wise it just takes too long. Also focus on the good minerals, palladium, platinum, gold, silver etc.
Mine 6 or 7 chunks and then scoop them all.
I had an interesting experience last night,
I've spent the last few nights flying all over the place looking for decent material to mine, but it's just been endless gallite/bauxite/rubbish - to the point where I was about to sell my mining kit,
Then last night I found a cluster of 4 metal-rich rings, found a good 4-5 asteroids which were dropping palladium/gold/silver, I have a 7 bin refinery and a type-6, I got about 13 palladium, 6 gold and 12 silver - flew back sold the lot for around 250k
I'd also read on the forums that the resources in the asteroids are *random* so I flew back expecting it to just drop bauxite, but low and behold - the same asteroids were still dropping palladium/gold/silver, so I did it several times before I went to bed, made some serious money..
Totally worth it - in my type-6 stripped out with 112 capacity, I could make about 90k on cargo runs shifting palladium/indum/gold, but if you find asteroids which drop valuable metal - you can make some serious money 8)