Time is money.
Average cost of incarcerating someone per day is $94.82.
6000000 divided by $94.82 per day is 63,277 days.
Which comes out to 173 years.
If They fleeced 6 mil, there's your sentence. You can decrease by a gratuity factor for making the prison population aware of what they are in for.
I mean. Seems only fair. We shouldn't facilitate taking advantage of captive populations. Might even help with the attractiveness of financial crime.