From technical perspective: you are not able to change your base price condition value from the document header most probably, because it is not set up to accept changes on header level in V/06.
From functional and business process perspective: I would definitely question the client why the price for these three materials changes so often and why it should be modified per document by end users instead of using pricing condition records for that purpose.
I wonder whether this is the most optimal design for the business process.
In addition to that - this means changing a price, which was most probably agreed with the client and it was used to execute credit check and/or trigger a request for advance payment. That has to be aligned with the customer again (out of the system), so this manual base price adjustment should be probably treated as a rare exception, not as a rule and there is no significant benefit to simplify it.
In most companies the users are not even allowed to adjust the base price in the sales orders, if this action is needed, it is done at quotation stage and may go through approval if the margin falls below certain agreed levels.