Addition in Table

Hi,
I am new in Databases, facing lots of problems. Right now my problem is :

I am using Access 97, I have two tables namely "PRODUCT" and "CUSTOMER". in the CUSTOMER table i have fields like ( Id,Name,Purchased Product Id, Quantity Purchased, Cost ).
I am not using Forms.
What i want is when i insert value in 'Quantity Purchased', 'Cost ' field should be automatically update, by taking Price from PRODUCT table and multiplying it by 'Quantity Purchased'.
will it be possible in Access. if yes plz guide me.

Thanks.