<select name="rpcalc" id="calc_s" onchange="calc_p(document.getElementById('calc_g').value)">
<option value="0" selected="selected">Выбрать...</option>
<option value="1500">первый</option>
<option value="7200">второй</option></select>
<a href="#" onclick="www();return false;">Купить</a></span></div>

function www() {
var idSet  = vSet.selectedIndex;
switch (idSet){
case 1:
var www = 'http://www.site1.ru';
window.open(href=www);
break; 
case 2: 
var www = 'http://www.site2.ru';
window.open(href=www);
break; 
}}

function www() {
	var index = document.getElementById('calc_s').selectedIndex,
			uri;
	
	switch(index) {
		case 0:
			uri = '#0';
			break;
		case 1:
			uri = '#1';
			break;
		case 2:
			uri = '#2';
			break;
		default:
			uri = '#';
			break;
	}
	
	window.open(uri);
}