Using a formula in a Purchase Order Template

Using a formula in a Purchase Order Template

At the moment under Total I have ${PurchaseOrders.Qty}*${Products.Unit Cost} which does not work. Is it possible to use such a formula within the inventory templates. I have tried to use brackets and an equal sign without any success. I cant use the default ${PurchaseOrders.Total} because I am using the products unit cost and not the list price.