To be honest, I would be interested in the answers from other players too.
Throughout the whole of my life in Eve-O, I've logged on for at least two hours a day (on average) and that is the guide to which you should measure yourself against when comparing the numbers below.
As a miner, I could earn about 3 million an hour. Mining bores me so much that I can not put any attention into it to improve this value. I have yet to play with null-sec mining, but I suspect I would be able to make 30 million an hour. (~= 160 million/month.)
As a tech 1 manufacturer, I failed to earn more than 4 million an hour, even with my mining fueling the ever hungry BPOs. However, this is not a very time consuming job so can be done alongside other things. (~= 184 million/month.)
As an inventor and tech 2 manufacturer, I do earn 1 billion in a week on good weeks. (Yes, that is a billion, not a million!) With material costs, however, profit is rarely more than 250 million per week. Selling the datacores from 4x lvl 4 R&D agents should net me 70 million a month, but I use the datacores instead. (~= 1 billion/month.)
As an industrial services provider, I've earned an average of 10 million a week. People will pay for anything! If I could face the high-sec courier runs in a slow-as-treacle freighter, then I could treble this. (~= 40 million/month.)
As an explorer, I can earn between 250 ISK and 100 million every week. Like invention, it is very random. (~= 1000/month -> 400 million/month.)
I've never played as a mercenary or pirate (inc. ninja, scammer, war-deccer, etc.). I hear they make more ISK than an explorer, but still not regular.
As a lvl 4 mission bear, I earn 100 million a week. Combining this with some of the other professions I mention here pays for some half-decent toys, most of which I only get to own for a week or two. ;) (~= 400 million/month.)
As a 23/7 wormhole denizen, I earned 500 million to a billion per week. Much of that was mining roids and sleepers, however, and the losses from tengus and lokis decloaking and taking you out (along with POS fueling costs) cancel out most of those profits. (Large WH corp, greater situational awareness, less profits. Small WH corp, less situational awareness but slightly greater profits given the losses you suffer!) (~=1 billion/month.)
As a null-sec denizen, I break even every week, and that includes the profits from ratting, plexing, tech 2 manufactuiring, industrial services and hisec missions. But then I am having more fun! (~= 0/month.)
The bottom line is really that fun is much more important than ISK (unless you need the ISK to buy your monthly PLEX) and anyone who hoards ISK can't be having much fun.