Ryu ga Gotoku Zero (released in Japan on March 12th 2015) is a canonical prequel to the first Ryu ga Gotoku game. Set at the end of the "Bubble" era of Japan in the late 80s, at the end of the Showa era and on the eve of the Heisei era, we play as 2 iconic characters from the series - Kiryu Kazuma (who is protagnist in all the other main series games) and Majima Goro (who was previously playable in the "Of the End" spin-off).
Kiryu is a member of the Dojima Family, part of the Tojo Clan. He became a Yakuza so he could follow Kazama Shintaro, despite Kazama's objections. Part of his work is collecting debts, but one day he is accused of murdering one of the people from whom he collected money. This results in a big fuss within the Dojima Family, who are looking to get their hands on a certain plot of land and that land happened to be where Kiryu supposedly killed someone. Kuze, one of the "big 3" within the Dojima Family, tries to make a deal with Kiryu whereby he would be excused if he spies on Kazama to get certain info regarding the plot of land. Kiryu, refusing to betray his beloved Kazama, ends up begging the Dojima Family to excommunicate him. Following his excommunication, Kiryu gets involved with Tachibana, who was acquainted with Kazama, and finds himself dragged into the issue surrounding the plot of land and the fighting that is going on over it...
Majima, formerly a member of the Shimano Family of the Tojo Clan, is currently in Sotenbori, Osaka, living an honest life and acting as manager of the big hostess club "Grand", under the surveillance of the Omi Alliance. Majima and Saejima had arranged the Ueno shooting, but Majima was prevented from going, though he tried to go against the orders. Because he opposed his boss, he was punished. However, Shimano entrusted Majima to Sagawa, of the Omi Alliance (who is a "sworn brother" of Shimano, despite being from different groups - similar to the relationship between Aoyama and Watase in Yakuza 5) for reasons even Sagawa doesn't know himself, meaning Sotenbori is now Majima's prison. However, Majima has a strong desire to return to the Tojo Clan and Sagawa lays down an order for him to allow that to happen - He must kill someone named 'Makimura Makoto'...
This is how you break the game and get insane amounts of money (way more than you'll ever need).
After getting the "Thank You, Extortionist" upgrade via CP Exchange, you can start making serious money. Change the difficulty to EASY (because heat actions do a lot more damage in easy) via the options screen, then get into a fight with an extortionist (those giant guys). Firstly, let him defeat you and take all your money. Next, find him again and defeat him - equip a weapon like a knife (bring plenty of them just in case) and spam the hell out of weapon heat actions (so, bring plenty of heat restoring items like staminas or tauriners). When you defeat him, you get a LOT of money. This method basically multiplies your own money - by letting him take it from you then winning it back from him plus a lot more on top of it. The more you do this, the more money you'll end up getting from him. You can continue multiplying your money forever using this method. I strongly recommend getting the Encounter Finder item (you get it from substory 23, when you select the black box at the end), so you can find these guys on the map easily.
It's possible to do this even on LEGEND difficulty - You need ability number 16 on Kiryu's Thug fighting style grid, which greatly boosts heat action damage while intoxicated. You can only gain this ability after completing the Pleasure Tycoon's story in Money Island. Then make sure you have a mix of alcohol and heat restoring items in your inventory. The massive boost of damage heat actions do when you are intoxicated means it's possible to money farm in the same way you can on Easy.
Sadly, because Majima cannot gain the "Thank You, Extortionist" upgrade, this method of money farming only works for Kiryu. It's harder to get money with Majima because of this (you just have to keep fighting extortionists and gaining smaller amounts of money from them as well as doing the Nightlife Island stuff). However, using the guy in the green suit (you meet in Chapter 6) and the guitarist (you meet in Chapter 7), you can transfer money between the 2 characters. So it's possible to do lots of money farming with Kiryu and then transfer that money over to Majima (though you must max out the friendship gauges of them both - meaning you can't exchange as much money as you want until right near the end of the game). Select the top option with the green suit guy to deposit money (you find him in Vincent - the bar where you can play darts and billiards - after meeting him). Then with Majima talk to the guitarist (outside club Grand) and select the second option to withdraw the money.
Like Kiryu, Majima also has an ability which greatly boosts heat action damage, but his boost comes while he's in 3rd gear heat rather than intoxicated. So, when using items to restore heat make sure you use 2 of them to get you up to 3rd gear or you won't do much damage at all.