NOTE: This involves editing a game file so make a back-up before proceeding.
Look for the file initialpref.* in the drive letter:\program files\infogrames interactive\monopoly tycoon\ directory or your custom install path. Each scenario folder contains the file initalpref.*
Use a text editor to open the file (in each scenario folder) and you can change the amount of starting money for you and your computer opponents on easy, normal, and hard.
Warning about the .initalpref file: When you edit the initalpref file, the amount of money you typed in will be given to the computer players, too.