map of material , how to back track it that how it changed

Hi Friends,

For material A . 

Price on date 1/6/2013 =292.69 Euro

Price on date 2/6/2013=139 Euro

user is asking me  what caused the price change.

This is related to moving avarage price of material . But how can I explan him for particular day means on 2/6/2013 how it became 139 Euro. with what supports.

Kr,

Mahesh