Rewriting a query to be more efficient
You could try to use top (1) with ties.

SELECT TOP (1) WITH TIES 
  SUM(OrderLine.Amount * Product.Price) AS OrderLinePrice, 
  Orders.Id, 
  Customer.Lastname AS CustomerLastName
FROM OrderLine, Product, Orders, Customer
WHERE OrderLine.ProductId = Product.Id
  AND Orders.Id = OrderLine.OrderId
  AND Customer.Id = Orders.CustomerId
GROUP BY Orders.Id, Customer.Lastname
ORDER BY OrderLinePrice DESC