If it doesn't let you order on the resulting TotalCost field then try nesting like this:

select *
  from (SELECT Products.ProductName,
               SUM(Order_Details.Quantity),
               SUM(Order_Details.Quantity * Order_Details.Unit_Price * (1-Order_Details.Discount)) AS TotalCost
          FROM Products,
               Order_Details 
         GROUP BY Products.ProductName)
 order by TotalCost