Update price in frontend from textbox

How can I update the price in the frontend form, when I type into a custom textfield?
Is there a javascript function? There is a select field, but a customer should have the
possibility to choose a free amount if he not want on of the selectbox values.

artofstyle.webjoy.dk/index.php/produkter...47-gavekort-2000-dkk

Maybe, there is a way to create a product where the costumer choose a free amount to a giftcard in the frontend?

Joomla 3.2 / Chrome/ php 5.4 / shop new