You can earn 400k in 30 minutes at the Island (click for tutorial)
You can also earn lots of bells by buying items cheap and selling them to other players at a higher price.
Or you can breed hybrids and sell them.
Or you could trade forum bells for in game bells.
Or you can sell villagers to other players.
I currently have around 300 million in my main towns mayors bank account and 99k on hand and a couple of million on each of my side characters plus a few thousand on hand. In my second town I think the mayor has 2 million in the abd. I spend lots of time buying and selling turnips with my friends, especially one friend who has four towns and TT on two of them. I have three towns so we check who has good prices and can sell full lockers all day.
I have about 16 million bells in my account. Now since I sold turnips and haven't deposited the bells in my account yet I don't know how much i'll have.