Modification Virtuemart/VMProduct Builder

I need to have a small modification made to a virtuemart 2.5/ vmproduct builder / textinput (plugin). Virtuemart has a custom attribute feature (which we make use of) that allows us to charge a price per letter that is typed into the text block. Currently, on the vmproduct builder page there is a total at the bottom of the page in the total price block. (see attachment). Currently, the total price for the content (character count) of the textblock only gets added when the "add to cart' button is pressed then the total (which includes the cost for the letters typed) are then shown on the cart page. The modification that is required must update the total at the bottom of the product builder page in real time as the text is typed into the text block. A character count must also be shown just below the text block. This should be a relatively easy / quick modification to those experienced with joomla / vmproduct builder / textinput plugin.