So first off, this guide assumes you're in endless mode and have already gotten a bit of cash.

So how do you gain insane amounts of personal wealth?
It's easy. Get yourself a mansion and fill it up with some stuff, the more expensive the better. Now with your decked out mansion host a party. If all goes well you should get a 25%, 50%, or 75% housing cost reduction (Its ok if you didn't, just wait until you can host another party).
Using this, buy as many mansions or other homes as you can. Now go home and play the next day as normal until the market closes.

Next Day:
Now the prices of home have returned to normal, which means so do the sell prices. Sell all the homes you just bought to make insane amounts of money.

And that's it. I got about 150M from 28M so its extremely op and can scale massively. Its very good for getting friendship related achievements.