Use the following trick on the first two discs for easy money. Go the worker's pier and move crates. When you shift is done, leave then go back in. You can do this three to four times in one day.
Go behind warehouse #8 on Fortunes' Pier (the one with the red doors). There is a set of Lucky Hit tables there. Choose the second table from the left. The board features a tiger with wings, and the bet is $20 to win $60. You should drop the marble fairly close to the left side of the table and win about 60% of the time. You can easily make $1,000 in a day.
Go to warehouse #8 after work. Win until you have $50, then go to warehouse #13 and win until you have $100 or more. Go to warehouse #10. When you are asked to place a bid, choose the one with 4 times (the bottom one), but do not press A yet. There is a turning triangle in the bottom right. Wait until it turns to a straight line, then press A. You will win most of the time. It is possible to win over $13,000 after one hour of playing.
To safely make a lot of money, save before you gamble. If you lose, use the in game reset then reload your game and try again. The best places to use this tactic are Wan Chai: Roll it on top, Lucky charm Qr near Three Blades street; Kowloon: Big or Small, Old Government Building, Dim Sum Qr. (Go through the underground walkway).
To save money on Disc 1, because you will be robbed go to a peddler on the street and spend all of your money on Zippo lighters. When you get robbed. you will only lose 16 yen. Later, after getting your room at the Come Over Guest House and your first day of work, go to the Pawn Shop. Pawn off your entire collection of characters, toys, and lighters. You will make 295 yen back, and thus 40% of your money (losing only 205 yen). This will help you survive the cost of boarding until you get past Disc 1.
To make money in Kowloon, just go to the acrobat style fighter. Run at him and when "B" appears at the bottom of the screen, press B to throw him. He will automatically lose all his health and you will have won the fight. You win more money every time.