Hi--
I assume you are using requisition management? To my knowledge, there is no built in way to have the items sort. I am sure there is some logic in terms of how the items are passed to the PO, but I do not know a way to control that. One way you might be able to do it is to manipulate the purchase order report so that they print in value order...not sure if that would help you?
Or maybe a Business Portal developer/customizer will find your post, as they might have a better suggestion.
Take care,
Christina