[Riku]

Something like this?

<script src="http://code.jquery.com/jquery-latest.js"></script>

<select>
<option id="country_and_cost">Select country</option> 
<option value="27">Argentina</option> 
<option value="95">Australia</option> 
<option value="150">Austria</option>
</select>

<input type="text" id="cost" readonly="readonly" value="" />

<script type="text/javascript">
// <![CDATA[
	$(document).ready( function() { 
	var selection = 0;
	$('select').change(function() {
	selection = $(this).val();
	$('#cost').val('Cost = ' + selection);
	});
	})
// ]]>
</script>

Hope this helps.

-Riku