Locate the file "game.cfg" in the config folder of the game's installation directory. Open the file with Notepad and search for the word "money". Change the amount to 999999999 for max money.
Get Your Money Back
Go into the building mode and build a long wall. Delete the wall and build it a second time. This time the wall won't cost anything and if you delete it now you'll get the money that it would have cost you to build it back. Repeat this a few times until you have all the money you need.
(supplied by: McKack)