I doubt very seriously that Axl is worth 150 million.
If he ever earned that much, he probably wasted it all recording Chinese Democracy.
I agree that sounds pretty dubious though. I'm sure he's made a comfortable living from royalties over the years, but there's no way he's currently worth that much. You don't usually make $150m from doing nothing for 20 years.
While I'm inclined to agree that he's not worth that much, Guns N Roses songs from their first 3 albums get played on modern rock and classic rock radio stations all the time. Remember radio?
They're still around, and still give out royalties. Sweet Child O' Mine is the most overplayed song in the last 25 years, beating out Smells Like Teen Spirit and that Puddle of Mudd song.