Add a sub-rule named Property to the World rule:

A Square may be Owned by a Player. That is listed in the [[World]] besides the Square listing. A Square which is owned by a Player is Color Coded Red. A Red Square which is not Owned by any Player loses the Red Color.

A Player in a Square which is not Red may Acquire that Square, becoming its Owner. To do so, e must spend an amount of B$ equal to X times the number of that Square, where X is equal 1 plus the number of color codes (including White) on that Square. To this purpose, the Go Square is considered to have a number 1 unit greater than the last Square in the World.

Third Proposal as allowed by Black-Out.