How To Update A Var Value

I am trying to get the value of a string in a div that is updated when a selection from a dropdown is chosen and assign that to a var.

When the page is created I have:
<div id="product_price">$123</div>

When a selection is made from the dropdown I change the display price with a function that returns:

document.getElementById('product_price').innerHTML = currency_symbol+price_format(price < 0 ? 0 : price);

I need to have a var that is updated when the price changes.So var prod_price = document.getElementById('product_price').innerHTML needs to be updated as well.The var is set fine when the page is crated but does not update when the dropdown is selected.