This is an unusual way to set this up. Normally, you select an item from a value list where the first field is a (usually) hidden ID number and the second field is a product name or description. Selecting the product by name enters the ID number into the field and links it to a related table of products where you have one record for each product with a unit price field. A relationship combined with an auto-enter field option then copies the current price for that item from the table of products into the table of line items in a portal on your invoices layout. You can see an example of this in the invoices starter solutions that come with your copy of either FileMaker 11 or 12.
Using what you have, you could define a relationship like this:
Entry::item = Items::Item
And then you can use the relationship to access Items::Cost once you have selected an item in your value list. But it's really not the best approach to use here.