SELECT ISNULL(ProductMixes.ClientCode, '') As Client
  , ISNULL(ProductMixes.ProjectCode, '') AS Project
  , ProductMixes.ProductCode
  , ROUND(SUM(ProductMaterialWeights.BatchWeight * PlantMaterialCosts.DeliveredCost /1000), 2) AS Cost
FROM ProductMixes 
  LEFT JOIN ProductMaterialWeights 
    ON  ProductMixes.DesignProductRelationUniqueID = ProductMaterialWeights.DesignProductRelationUniqueID
    AND  ProductMixes.ProductCode = ProductMaterialWeights.ProductCode
    AND ProductMixes.Version = ProductMaterialWeights.MixVersion
  LEFT JOIN PlantMaterialCosts 
    ON  ProductMixes.Plant = PlantMaterialCosts.PlantCode 
    AND ProductMaterialWeights.MaterialCode = PlantMaterialCosts.MaterialCode
WHERE ProductMixes.Plant = 'XAT'
  AND ProductMixes.Active = 1
GROUP BY ISNULL(ProductMixes.ClientCode, '')
  , ISNULL(ProductMixes.ProjectCode, '')
  , ProductMixes.ProductCode
ORDER BY ISNULL(ProductMixes.ClientCode, '')
  , ISNULL(ProductMixes.ProjectCode, '')
  , ProductMixes.ProductCode