Filling input field based on dropdown menu selection with Javascript

HTML:

<select>
    <option>Select an item</option>
    <option data-price="20.00" value="1">Item 1</option>
    <option data-price="15.00" value="2">Item 2</option>
    <option data-price="24.00" value="3">Item 3</option>
</select>

<input type="text" id="price">

​ JavaScript (using jQuery):

​$('select').on('change', function() {
    $("select option:selected").each(function () {
        $('#price').val('$' + $(this).attr('data-price'));
    });    
});​​​​​​​​

And see this jsFiddle for a working example: http://jsfiddle.net/chnUn/

Using jQuery, you can do something like this:

$("#DropDownId").change(function ()
{
    var selectedItem = $(this).val();

    if (selectedItem != null && selectedItem != '')
    {
          $("#InputId").val(selectedItem);
    }      
});

Get all the things you need to display from your server (preferably in JSON)

and generate your select elements like this..

<select id="mySelect">
<option name="item1ID" defaultPrice="defaultPrice1" >ItemName1</option>
<option name="item2ID" defaultPrice="defaultPrice2" >ItemName2</option>
<option name="item3ID" defaultPrice="defaultPrice3" >ItemName3</option>
<option name="item4ID" defaultPrice="defaultPrice4" >ItemName4</option>
<option name="item5ID" defaultPrice="defaultPrice5" >ItemName5</option>
</select>

You can get default price like

$("#mySelect").on('change',function(){
alert($(this).attr('defaultPrice');
});

Whats your server side language?

I would just do something like this:

<script language="javascript">
var items = new Array();

<?
$res = mysql_query("SELECT id, name, price FROM ITEM_TABLE");

while($item_object = mysql_fetch_object($res)) {
    ?>
        items[<? echo $item_object->id; ?>] = "<? echo $item_object->price; ?>";
        ('#item_drop_down').append('<option value=' + <? echo $item_object->id; ?> + '>' + <? echo $item_object->name; ?> + '</option>');
    <?
}

?>

('#item_drop_down').Change('#item_price_input').text(items[('#item_drop_down').value()]);

</script>

where the drop down is item_drop_down and the input you want the price in is item_price_input.

Create Select dropdown menu like below where option value = price of item

<select name="item_list" id="item_list" onchange="update_txt()">
<option value="">Select item</option>
<option value="10"> item1</option>
<option value="20">item2</option>
</select>
<input type="text" id="item_price" value="">

JS:

function update_txt(){
price = document.getElementById('item_list').selectedIndex.value;
document.getElementById('item_price').value = price;
}