Create plugin for woocommerce

I have just create a plugin for WC (add a new tab on setting WC) but now i need to add, ony for backend 3 custom field.

One for add gr
One for add working price
One for add % service

The finnaly price must be a result from:

product price (es 40) + field work price (es 7) x gr field (es 5) + % serve (es 40%)

So finnaly price must be (40 + 7)=47x5=235+40%=329