Believe it or not, it's actually possible to get more than $1M. You can get as much as $4.8M (yes, you read that right) if you happen to get the following chain of events:
- first 23 cases chosen aren't 750K or 1M
- the lucky case is chosen as the third to last case (1 in 26 chance), and the offer multiplier minigame is activated (1 in 2 chance)
- the player selects the 5x multiplier (1 in 5 chance - the others are 1x (weight: 2), 1.5x, and 2x (both weighted at 1))
The banker will then attempt to offer $880,000, but with the 5x mult this instead becomes $4,400,000 (both of these are Diamond Tacos).
The chance of this happening is (I believe) 2/(26 choose 2) * 1/26 * 1/2 * 1/5 = 1 in 42,250
However, there are many other ways to get above $1M. I would estimate, if you added all these ways together, the total probability would be ~1 in 5,000 - but this strategy requires risky gameplay and has quite possibly never been achieved.
It's much, much easier to get less than nothing though - I've done that a few times myself. I think my personal record is -$22K, but you can theoretically get offers as low as -$90K with bad luck on the +/- minigame.