not really enough detail to provide query ideas - but assuming your productid, price, quantity and supplierID is in your orderdetails table, join the table to itself on the productid and the yearmonth of the orderdate. you can then compare prices.
Something like
Code:
SELECT T1.ProductID, T1.SupplierID, T1.Price, T2.SupplierID, T2.Price, T2.Quantity, (T1.Price-T2.Price)*T2.Quantity AS Saving
FROM tblOrderDetails T1 INNER JOIN tblOrderDetails T2 ON T1.ProductID=T2.ProductID AND format(T1.OrderDate,"yyyymm")=format(T2.OrderDate,"yyyymm")
WHERE T1.SupplierID<>T2.SupplierID AND T2.Price<T1.Price
This in itself is probably insufficient since it is only going to return products bought from more than one supplier in a month. but perhaps gives you an idea on a way to go.