At maximum so far, it seems to be that the minimum Sunday buy is 90 and the maximum week price is 650.
90 -> 650 ~ 7.2x price = 620% profit
If you move that conservatively, (100 buy, 500 sell)
100 -> 500 = 5x price = 400% profit
And if you assume that you start at even 200k bells
200k -> 1m -> 5m -> 25m -> 125m -> 625m -> 999m (max)
And with that mathed out it starts to become obvious that taking 10+ trips to anyone's island is tedious, but not as tedious as storing hundreds/thousands of stacks of turnips.
And the only reason I type that out is because even as far back as Wild World on the DS there were items you got for storing certain amounts of bells in the bank, all the way up to 999,999,999 (Which gave you the Town Model). I don't know much about the other games so there may be no reason in this one (Maybe it's already been datamined?)
The reality is that after 10m, it's probably best to stick to your own town and get the best price you can, hopefully doubling money each time. Otherwise you're doing some pretty tedious island hopping at approximately 3trips/1million bells invested
EDIT: Here it is. The last reward in New Leaf was at 100,000,000. So they toned it down from the 1 billion of previous games lol
The savings account is a feature in the Animal Crossing series that allows the player to deposit or withdraw their Bells. It is accessed either from Pelly or Phyllis...
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