Am trying to add a discount on the product price depending on where are the users are from "filled in the registration form"

Am thinking about fetching the data "state" via hook and altering the discount rate depending on the fetched data.

but I can't figure out where can I set the value of the discount and change the price of an item.

I don't think the rules will be much of a help, since all be adding a lot of conditions to it.

Thanks