What part of the javascript are you missing?
How to call a function on change?
<select onchange="functionName()">
How to refer to a list?
document.getElementById('listName')
How to refer to a selected value in a list?
list.options[list.selectedIndex].value
How to refer to a layer?
document.getElementById('layerName')
How to set the contents of that layer?
layerName.innerHTML = "$" + totalInDollars + ".00"