We're talking 100k to 400k yen per fight, depending on what you do and how you do it.

Light spoilers ahead up to the fourth palace including party members (though I'll refer to them by their codenames to lessen the impact).

What you need:

1) Joker with a wind (Garu) and nuclear (Frei) spell. Individual, not all-hitting. Preferably on the same persona.

2) A persona with Heat Up and/or Invigorate 3, preferably both, meaning you can repeatedly recover SP). If it can heal too (Mediarama or Mediarahan work, Salvation would be better) it's a bonus.

3) Queen, Fox and Panther in the party. They have the most bullets combined. I think you can figure out the rest yourselves, but here we go regardless. Panther needs the SP Adhesive accessory (Death confidant level 7 is needed).

From the Corridor Middle safe room, go straight ahead and fight the first enemy you see. If you don't see the enemy Anzu among the ones you need to fight, run away (but make sure to win some ambush battles to keep the alert level low, this is important to maximize the encounter rate of Anzu. Anzu is weak to bullets. Fox and Queen should get rid of their bullets first, then Panther, then Joker. Kill everything except one Anzu and shoot him once. Hold Up happens. Keep asking for money. He'll dish out 1000~ to 10000~ per ask. At some point he'll get angry. Now the battle can vary in one of three manners:

1) if he tries to fight back: who cares, shoot him again. Repeat Hold Up.

2) he summons help. He'll only ever summon Naga, which is weak to wind, or another Anzu, which is weak to nuclear. Kill the Naga or the Anzu with the least health and repeat Hold Up. Unless...

3) he summons help and the help wants to run. This can go two ways: either you kill the help immediately and go back to blasting, or Panther uses the random "Sexy Technique" passive skill, which robs the new enemy of its turn and its intention to run. Repeat #2.

Now what if the remaining Anzu is the one that wants to flee? You use Dormina with Panther. Anzu will fall asleep... And recover some HP, which is good because you want him to be durable and take every single bullet. Make sure you shoot him while he's sleeping or he'll run the moment he wakes up. To further help with this, make sure you equip everyone's weakest, fullest and most precise gun.

I'm currently sitting pretty at about 2 and a half million and am missing only two personas from the entirety of the available compendium (some personas are locked behind certain dates, requirements and social links).

Have fun!