SOURCE

   var oUl=document.getElementById("list");
    var aLilist=oUl.getElementsByTagName("li");
    var totalMon=document.getElementsByClassName("totalMon");
    var summary=document.getElementById("summary");
    var aStrong=document.getElementsByTagName("strong");
    var aUnit=document.getElementsByClassName("unit");

    var sEm=summary.getElementsByTagName("em");


    //总计
    function count(){
        var pAll = 0;
        var aPrice = 0;
        var temp = 0;
        for(var i=0;i<aStrong.length;i++){
            pAll+=parseFloat(aStrong[i].innerText);
            aPrice+=parseFloat(totalMon[i].innerText);
            //最贵单价
            if(aStrong[i].innerHTML != 0){
                if( parseFloat( aUnit[i].innerHTML ) > temp ){
                    temp = parseFloat(aUnit[i].innerHTML);

                }
            }
        }
        //最贵单价
        sEm[2].innerText=temp;
        //商品合计个数
        sEm[0].innerText=pAll;
        //总计花费
        sEm[1].innerText=aPrice;

    }

    //数字增减
    for(var i=0;i<aLilist.length;i++){
        cart(aLilist[i]);
    }

    function cart(ele){
        var aLi=ele;
        var aIpu=aLi.getElementsByTagName("input")[0];
        var aIpd=aLi.getElementsByTagName("input")[1];
        var aEmp=aLi.getElementsByTagName("em")[0];
        var aEmd=aLi.getElementsByTagName("em")[1];
        var aStrong=aLi.getElementsByTagName("strong")[0];


        var num=0;
        aIpu.onclick=function () {
            num++;
            aStrong.innerText=num;
            aEmd.innerText=num*parseFloat(aEmp.innerText)+"元";
            count();

        };
        aIpd.onclick=function () {
            if(num>0){
                num--;
            }
            aStrong.innerText=num;
            aEmd.innerText=num*parseFloat(aEmp.innerText)+"元";
            count();

        };

    }

<ul id="list">
    <li>
        <input type="button" value="+"/>
        <strong>0</strong>
        <input type="button" value="-"/>
        单价：<em class="unit">12.5元</em>
        合计：<em class="totalMon">0元</em>
    </li>
    <li>
        <input type="button" value="+"/>
        <strong>0</strong>
        <input type="button" value="-"/>
        单价：<em class="unit">12元</em>
        合计：<em class="totalMon">0元</em>
    </li>
    <li>
        <input type="button" value="+"/>
        <strong>0</strong>
        <input type="button" value="-"/>
        单价：<em class="unit">22.5元</em>
        合计：<em class="totalMon">0元</em>
    </li>
    <li>
        <input type="button" value="+"/>
        <strong>0</strong>
        <input type="button" value="-"/>
        单价：<em class="unit">4.5元</em>
        合计：<em class="totalMon">0元</em>
    </li>

    <li>
        <input type="button" value="+"/>
        <strong>0</strong>
        <input type="button" value="-"/>
        单价：<em class="unit">55元</em>
        合计：<em class="totalMon">0元</em>
    </li>
</ul>
<div>
    <p id="summary">
        商品合计共：<em>0</em>件，共花费了：<em>0</em>元<br />
        其中最贵的商品单价是：<em>0</em>元
    </p>
</div>