1900 is what I came up with the other day and 133 cubic inches. Even a small size attaché easily had that cubic capacity, iirc. (I might have used a website called "Briefcase Full of Money" or a similar one for the bill dimensions.)
I think the "adequate size attaché" is a riff on the stipulation in
Ruthless People that the notes be delivered in a black American Tourister briefcase model number



. The Ramsey ransom note writer seems to have used the ransom demand in
Ruthless People as something of a template: "Any deviation of my instructions will result in the immediate execution of your daughter" is probably too close to "If you deviate from our instructions in any way whatsoever, she will be killed" to have happened by chance.
That last part always bothered me. Did Patsy have the RP ransom demand memorized? Did she pop
Ruthless People in her VCR because she suddenly needed a ransom note template? Because of its complexity, I've come to conclude that she did prep work on the ransom note. (Probably one of the few things I agree with Lou Smit about.)