What Formula Should I Use For Mobsters Game?

Hey guys, I've been playing iMobsters (iPhone game) which is the same as Mobsters and Mafia Wars on Facebook and Myspace. I need a formula that I can use, so I can spend my money efficiently on real estate. (you know, like how many of whatever type of real estate I should buy) Because what is happening is that I have bought a lot of high dollar real estate properties and not enough lower priced real estate to add up to my high priced ones.