js表单计算金额问题

<table width="600" border="1" align="center" style="text-align:center;">

  <tr>

    <td width="102"><input type="checkbox" id="all"/>全选</td>

    <td width="203">商品</td>

    <td width="119">单价</td>

    <td width="148">数量</td>

  </tr>

  <tr>

    <td><input type="checkbox" id="qianbi"/></td>

    <td>铅笔</td>

    <td>1.5元/支</td>

    <td><input type="text" id="qianbi_t" value="1" /></td>

  </tr>

  <tr>

    <td><input type="checkbox" id="yuanzhu" value="" /></td>

    <td>圆珠笔</td>

    <td>2.5元/支</td>

    <td><input type="text" id="yuanzhu_t" value="1" /></td>

  </tr>

  <tr>

    <td><input type="checkbox" id="gangbi" value="" /></td>

    <td>钢笔</td>

    <td>3元/支</td>

    <td><input type="text" id="gangbi_t" value="1" /></td>

  </tr>

  <tr>

    <td colspan="4">金额合计：<span id="m" style="color:#F00;"></span>元&nbsp;&nbsp;&nbsp;&nbsp;<button>提交订单</button></td>

  </tr>

</table>

<script type="text/javascript">

    var all=document.getElementById("all");

    var qianbi=document.getElementById("qianbi");

    var yuanzhubi=document.getElementById("yuanzhu");

    var gangbi=document.getElementById("gangbi");

    var qb_m=0,yzb_m=0,gb_m=0;

    all.onclick=function(){

        if(all.checked){

            qianbi.checked=true;

            yuanzhubi.checked=true;

            gangbi.checked=true;

        }

        else{

            qianbi.checked=false;

            yuanzhubi.checked=false;

            gangbi.checked=false;

        }

        sub();

    };

    qianbi.onclick=function(){sub()};

    yuanzhubi.onclick=function(){sub()};

    gangbi.onclick=function(){sub()};

    document.getElementById("qianbi_t").addEventListener("change",function(e){

        sub();

    });

    document.getElementById("yuanzhu_t").addEventListener("change",function(e){

        sub();

    });

    document.getElementById("gangbi_t").addEventListener("change",function(e){

        sub();

    });

    function sub(){

        if(qianbi.checked){

            qb_m=document.getElementById("qianbi_t").value*1.5

        }

        else{

            qb_m=0;

            all.checked=false;

        }

        if(yuanzhubi.checked){

            yzb_m=document.getElementById("yuanzhu_t").value*2.5

        }

        else{

            yzb_m=0;

            all.checked=false;

        }

        if(gangbi.checked){

            gb_m=document.getElementById("gangbi_t").value*3

        }

        else{

            gb_m=0;

            all.checked=false;

        }

        var sum=qb_m+yzb_m+gb_m;

        document.getElementById("m").innerHTML=sum;

    }

</script>