fruct455,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function(){
var input = $("input"), span = $('#res_sum'),ser_pr = [[300,100],[1000,150],[3000,200]] ;
input.on('input',function() {
val = +this.value || 0;
for (var i=0; i<ser_pr.length; i++) if(val < ser_pr[i][0]) {val *= ser_pr[i][1]; break}
span.html(val)
});
});
</script>
</head>
<body>
<input name=""><span id="res_sum"></span>
</body>
</html>