I think this may be due to a glitch and/or patch. Within a few days of the game coming out, I went into co-op in another Hero's world and got the Achievement in about 40 minutes to an hour. We didn't do any jobs/quests that made a bunch of money, as I recall. We just did the Demon Door, got married, etc. We didn't even form a business partnership.
Since then, I've had several Heros join my world to try to get the Achievement, and the result was what you state. They got about 10g every 5 minutes, meaning it would take almost 9 hours playing time to get the Achievement. We tried having them do a job and earn 1,000 gold. They earned the gold, but didn't get the Achievement. (suggesting that you have to earn the 1,000 in Henchman wages, not just earn it).
My only suggestion is to try to find someone who has a quest to do that earns money. Maybe that will work.