Seriously, what's the logic behind bills anyway?

I have fun moving Sims with low-level or no jobs into a mansion and giving them only $1000 to start with. It's fun making them scramble to earn money quickly or sell their belongings in order to keep the lights on. I'm not cruel to my Sims, but I enjoy the challenge of paying those hefty bills. The teens learn responsibility (in my mind, not necessarily through the Parenthood pack) when they help earn the money.