Stay_Strong,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script>
function calculate() {
var room_length_nan = document.getElementById('room_length'),
room_length = room_length_nan.value.replace(/\,/g,'.');
room_length_nan.value = room_length;
var room_width_nan = document.getElementById('room_width'),
room_width = room_width_nan.value.replace(/\,/g,'.');
room_width_nan.value = room_width;
var area = room_length * room_width,
price_ceiling_m = 299,
price_new = area * price_ceiling_m;
document.getElementById('price_new').value = price_new.toFixed(2) + " руб.";
}
</script>
</head>
<body>
<form action="http://">
<input id="room_length" name="">
<input id="room_width" name="">
<input id="price_new" name="" readonly="readonly">
<input name="" type="button" value="calculate" onclick="calculate()">
</form>
</body>
</html>