Linking fields ?

I have created a database for doing chargebacks and I have two fields
(item and cost) which I wish to link. Both these fields I have
configured as pop-up lists.  I would like to have the database
configured so that when a particular item is selected from the pop-up
list in the item field a pre-determined cost will be auotomatically
entered in the cost field.
Is this possible, I am using FM Pro 6.0.
Thanks in advance for any help.

Cheers David