<script  Language="JavaScript">
function calculate()
{
 answer.innerHTML=calc.a.value*calc.b.value;
}
</script>
<form name="calc"> 
<select name="a"> 
    <option value="13">рубероид</option> 
    <option value="8">пергамин</option> 
    <option value="99">изоэласт</option> 
    <option value="119">техноэласт</option> 
    <option value="35">бикрост</option> 
</select> 
<input type="text" value="1" name="b" size="12">м<sup>2</sup>&nbsp
<input type="button" value="Расчитать" onclick="calculate()"></form><hr><span id="answer">0</span> руб