I'm not sure which version of GP you are in. In GP 10 and up you can use the inventory adjust cost tool (it is unders tools>> Inventory). This will allow you to adjust cost for any receipt layer.
Frank's GP2themax blog has an entry on how the tool works. here is the link:
http://gp2themax.blogspot.com/2010/03/gptip42day-adjust-cost.html