Select j.JobId,
case
When Sum(p.Price*op.Quantity) is null then 0.00
else Sum(p.Price*op.Quantity)
end as [Sum]
from OrderParts as op
 Full join Parts as p on p.PartId= op.PartId
 full join Orders as o on o.OrderId=op.OrderId
 full join Jobs as j on j.JobId=o.JobId
 Where j.Status='Finished'
group By j.JobId
Order by Sum(p.Price*op.Quantity) DESC,JobId ASC