Payout is based on cop percentage.
If the server has 50/50 cops/criminals the payout is $20,000 - $30,000 for exploding the vault and $30,000 - $50,000 for hacking the vault.
This can decrease to 50% with no cops online, or increase up to 300% if there are 3x as many cops as criminals.
So the max payout for bank robbery is $150,000 per bag if there's a huge number of cops.
I'm crap at the game and I've robbed the bank many times with low effort. You have to just get lucky sometimes, and rob it at the same time as many others, or team up with people.
Eventually more robbery locations will be added which will make each individual robbery easier as the cops will be spread over multiple locations.
There are already planned improvements to the bank robbery to actually make it a bit more difficult. I'm rejecting this suggestion for now because I think the payouts are fine but I will review them in the future with other payouts when I look at the 'economy' as a whole.