In the pedias, you can find out how much a selected item is worth in Amazon Marketplace:
Choose the item in the pedia, go to Advanced Menu-> Amazon Marketplace.
This will bring up a window with the prices for items similar to yours. It is a great tool to use if you are planning on selling some of the items in your collection.
If you just want to know how much the collection is worth based on the price that each item has (this price is either set automatically by the site you downloaded the info from or added by the user), you can just choose a collection, then enable "Value" column under Preferences-> Style-> Status Bar View.