A Design Simoleon.
Design Simoleons are the currency used in City Design Challenges. When first starting a design challenge the player starts with 8,000 Design Simoleons. They can be earned by upgrading residential zones in Design Challenges can and be purchased for Simcash. They are used to replace regular Simoleons when doing design challenges. It requires 1/10th as many Design Simoleons as the service would normally cost with Simoleons. They are used to purchase services and upgrade roads in city design challanges.
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In the early acess mode of Design Challanges players would start with 15,000 Design Simoleons and get 15,000 every after challange however those were later taken away and replaced with 5,000. After that the 5,000 was replaced by 8,000.