I have created a custom field for a dollar amount of a product

THINGAMABOB is $27.99 a month recurring

I also added a RECURRING TOTAL field 

I want to create a field that will give the output when someone adds that item with a quantity

so it would look liek this

QTY - 15

THINGAMABOB - $27.99 (Recurring Monthly)

RECURRING MONTHLY TOTAL - $419.85 <----- how do I get that total into my quote?