Well I can't talk for other players, I can only say how much I can make. Say I'm in a hypothetical scenario of not having to study or work nor have a social life and I could farm 24 with 2PCs + run 2-3 kiosks (farming using low lvl chars). I would ONLY focus on making ecash and not upgrad the gear of my characters further and I would use some money/crone/ecash to invest into items that increase in value over time.
I would say I'd be able to make about 30-50k ecash a month in that specific scenario, depending on your luck (opening boxes) and also on getting sales (to buy IM items cheap/stock up on hks, som 7s etc for farmers)
If I'd however do the same as I said above but I somehow had a full party of 6 characters+50 with max gear. I'm pretty sure I could generate 50k ecash a month or more. You have to however take into account that the amount of ecash it costs to gear a party of 6 with max to semi max gears is very high (200k ecash for each char * 6 = 1.2 million ecash invested) this is your buy-in, after that you have to spend lots of money to maintain those characters so that's deducted from your income.