1. Nobody buys music these days and even 20 years ago, no local rapper was selling the kind of units that would allow the lifestyle that's being bragged about in tracks. And nobody is getting in the neighborhood of 100k for a live appearance either except for select few but it's not those artists that we're talking about
2. If you were making that kind of money through other means, legally or illegally, you most likely would not have much time to make music, because unless you're a billionaire with hundreds of people working for you and a lot of spare time (and you're not), you would have to spend a lot of your time doing what is making you money and would have little incentive to waste time on making rap records.
3. Artists these days put out dozens of albums and mixtapes a year. This can only be achieved by spending long ours in the studio making tracks, and in addition to that, the number of videos being made has increased significantly and it takes time to shoot those, they do shows, etc., etc., all time-consuming activities. They ain't getting rich off of music, so if they do have a lot of money it must be coming from somethings else they do (again, legal or illegal, it does not really matter that much, it's still work). But how is it possible that they have the time to get involved in such activity if they spend so much time making tracks? Add to that the additional constraint on their time imposed by the amount of drugs they brag about doing, which is the one truly believable thing they brag about - you're unlikely to be out there doing serious business when you're completely wasted - and it becomes really impossible to see where the money is coming from.
The only logical conclusions is that the money simply does not exist.