Cheap purchases:
Play against the CPU and land on an unowned property. Place the property up for auction. Keep bidding to win the auction as no other player can challenge your offer.

Cheat mode:
Enable the "Cheating is allowed" selection under the "Custom rules" option by clicking on it. Then, press [Plus] when rolling the dice to move one space forward or [Minus] to move one space back.

Cheat mode (WM edition):
Type gimmesheep during game play.

Unlimited money:
Type I'm Bill Gates, I'm VERY rich! (case-sensitive).