Its like loving impossible to get a sim in a brilliant financial position. Even at the top of the career they're still just low class. :L
I actually found it out in Sims 2. See, you need a biggish family. And if you're building a mansion, why not? You have enough space.
But you grow them all up and get them amazing jobs, get them to the top of their career. And when they die, just make sure you have another person to raise. :) You'll have an endless wave of money. Just like real-life rich families, let the family move on with life, let it evolve.
Oh and keep a fire alarm in the house, because you may accidentally kill them all like I did on Sims2. :c