Monitoring Cost Price Changes (Supplier Prices)

Hi to all.

I'm hoping somebody can point me in the right direction. I'm fairly certain i've seen something similar on the NS demo accounts.

I'm after creating either a search (or a report) that will show me monthly what cost prices (suppliers prices) have changed on which products so we can closely monitor what prices may need to be reviewed. I've noticed that when you manually change the supplier price within an inventory record, the changes of such are not noted under the system information notes, therefore no record of the previous prices are held on the system? I therefore fail to understand how the system can record the price differences between the old and new buy prices.