Formula between multiple forms

I'm unfamiliar with the scripting language and I'm creating a database for a non profit managing the inventory we maintain.

I created a form for the individual Inventory items. I'm on a second form an Order form where I added a lookup field linking to the Inventory item. Another field on the Inventory form is Cost.

I'm trying to create a formula field on the Order form that will add together the cost field from the inventory page for each Inventory item selected.

Making sense? Thank you!