[bes]

<form name="ya">

<select name="sistnalog">
	<option value="4000">
	<option value="3000" >
	<option value="5000" >
</select>

<select name="viddeat" >
	<option value="1.2">
	<option value="2.1" >
	<option value="2.2" >
	<option value="2.5" >
	<option value="1.1" >
</select>

<input type="button" value="рассчитать" onclick="alert(my())">
</form>

<script>
function my() {
	var s = parseFloat(ya.sistnalog.value);
	var m = parseFloat(ya.viddeat.value);
	return s * m;
}
</script>