How can I use personalized Products Field into Invoices Field?

How can I use personalized Products Field into Invoices Field?

Hello!
 
First of all, I need to configure as personalized:
In the Products fields
- Product Cost: U$ xx,00 (ok, easy)
 
so,
In the Invoice fields I'd like to see ${Products.Producst Cost (U$)} and then to apply a rule that if I check the List of Fields at Invoices called "Sell Type" it makes a "Back Comission Calc"... for example:
 
${Products.Unit Price} - ${Products.${Products.Custo (U$)} = X / "Sell Type" (that defines the correct % of back comission).
 
Can anybody help me?
Thanks!
Mauricio