After the game first released and bots were introduced, I was making €2000-3000 a month from running bots, renting servers to run more bots and just selling the gold on the RMAH.
They didn't give a damn about botters (still don't) and eventually it got out of hand, gold price dropped so little it wasn't worth it.
It was such a fun thing to do really..
I'd buy the game key for like $40, invest $50 in a decent VPS to run it off and the same for a bot license.. Outsource the leveling and gearing and i'd make my investment back in a week.
At my peak I had like three servers full of bots, once a week going through all of them to sell gold and stuff on the RMAH, totaling about €2500 average PROFIT per month (could've done more if I scaled up with more servers but I figured it wouldn't last).
It really died off at the start of this year, at least on a proper scale, since gold price dropped so much.
I know people still banked hard until the summer by providing "paragon leveling" services etc though.
Games like this are often goldmines, MMORPG's that is. I've banked in a few other games as well, with even bigger profits and know people who make 5 figures a month from simple stuff like this.