Custom field multiplying

[Tiligadis]

Hello im new to VM i am trying to make a custom field that when the product is added to the cart will be something like
Product Price X Customfield value X quantity

so for example a product price is 5euro and the custom field value is 2 and the quantity 5
is 5 X 2 X 5 = 50 when adding to cart.
Im using Vm 2.02 and J2.5

Thanks you
(sorry for my poor english)