1. 将var $inst = $('.inst_amount');移至使用位置:
   var total = 0,$inst = $('.inst_amount');-实际上甚至没有.查看我的代码

1. move var $inst = $('.inst_amount'); to where you use it:
   var total = 0,$inst = $('.inst_amount'); - actually don't even. See my code

使用逗号分隔变量

干-不要重复自己

正确提供该类，然后将该字段添加到容器中.

give the class correctly and add the field to a container.

function sumIt() {
  var total = 0, val;
  $('.inst_amount').each(function() {
    val = $(this).val();
    val = isNaN(val) || $.trim(val) === "" ? 0 : parseFloat(val);
    total += val;
  });
  $('#total_price').html(Math.round(total));
  $('#total_amount').val(Math.round(total));
}

$(function() {

  // $('.datepicker').datepicker(); // not needed for this test


  $("#add").on("click", function() {
    $("#container input").last()
      .before($("<input />").prop("class","inst_amount").val(0))
      .before("<br/>");
    sumIt();  
  });


  $(document).on('input', '.inst_amount', sumIt);
  sumIt() // run when loading
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="button" id="add" value="add" /><br/>
<form id="inst_form">
<div id="container">
  <input type="text" class="inst_amount" value="4" /><br/>
  <input type="text" class="inst_amount" value="" /><br/>
  <input type="text" id="total_amount" value="0" />
</form>
</div>
<span id="total_price"></span>