As the Shopping Helper refreshes the product data in your Shopping Lists, it will save the current prices to provide Price Histories for the products in your lists.
To open the price history viewer, right click on the product in the or the Sidebar and select the 'See Price History' choice from the pop-up context menu. This window will show a graph of the price history as well as the detail prices ordered by date. When the product is sold by multiple stores, the tabs can be used to see the prices for each individual store. At the top of this window you can select any of the products in your Shopping Lists.
You may disable the price history feature completely, which prevents any history data from being saved to your computer.
See the Preference Pane Section for how to access and change your Shopping Helper preference settings.