Backup the file you are about to edit before proceeding. Go to your savegame folder, and open savegame?.dat (where ? is the number for the saved game that you want to edit) in notepad.

Do a word search for 'money' (press Ctl-F and look for 'money'), and change the number beside it to 999999. Now save the file and run the game and you should have lots of money!

I'm not sure which versions this works on but others have reported success.