How to Make Money Fast
You need to have 2 BlackKingNumemon and one Digimon that can do great piercing damage. Go to Under Kowloon LVL 5 to the last area and use a function call to enter battle. If there is a KingEtemon then defeat him and escape he has a chance to drop Billionaire USBs. If you do not encounter him escape and repeat until you have all the USBs you need 6-9 (8 is ideal).
Typically you can take your PrinceMamemon to whatever area you have access to that is the highest level and battle there with them to earn yen. Odaiba is the best place before post-game grind money.
Post game however you can use the Abyss server. Make your way to the 12 floors and continue upwards. Your party must include 2 PrinceMamemon with all Billionaire USBs and 1 Digimon with a strong piercing skill like Gallantmon CM, Beelzemon BM, or AlPhamon Ouryuken (they should also have Billionaire USB. You should know there are various enemies in the abyss server with a great buff which makes it difficult to do much damage to them. If you can find one such enemy you can hit kill with your Digimon for example for Beelzemon and Virus pierces I recommend fighting Meramon on floors between 10 and 20 find him or another buffed Digimon kill them with your piercing and then escape the battle. Rinse repeat once you find the right floor. Using the above method nets around 300000 yen per fight.