If you have the time, review the webinar materials for "US ACCOUNTING - OPENERP V7.0"

There is a section "Cost Flows - Raw Materials, Work in Progress, Finished Goods" that shows a demo and discussed configuration needed to support capturing costs during Manufacturing.

There is a recording and a slide deck at http://www.ursainfosystems.com/webinar

Set the accounts for product categories to Raw Materials or Products for Sale, depending on the category