To get this trophy, simply get a property for 10% less than the original price (so get a $100 property for less than $90, for instance). This is done through auctions. Any time a player lands on a property, they can choose to buy it or auction it. All players participate in the auction. As the auction goes on, players can withdraw rather than bid.
Simply win the Auction while saving 10% on the original value of the property.