I got this achievement by the time I was level 11. I had 5 wheat farms, 2 quarries, and 1 vineyard(found this one in stash).
NOTE: ACHIEVEMENT POPS AT 100,000 NOT AT 50,000.
Wheat farms give you somewhere between 180 and 190 coins every 1 minute.
Quarries give you 280 coins in 1 minute 30
Vineyard gives me 237 coins every 1 minute
Headquarters gives 2025 coins every hour.
You need to tap the building when a coins symbol shows up in order to collect it. You can of course go for the achievement with more or fewer coin generating buildings, but this is the way I did it. You can build more buildings if you go to Tools-> Shop -> Income
You can see your progress if you press Tools-> Achievements.