textbox非数字不能输入/求和/保留两位小数

onKeyPress="fun()"

 <td style="height: 23px; width: 55px;" align="center"> 单　　价</td>                                   
                        <td style="height: 23px; width: 108px;">
                                <asp:TextBox ID="tbPrice"  Width="76px" runat="server" onKeyPress="JHshNumberText()" CssClass="input_text"></asp:TextBox>
                        </td>
                        <td  style="width:56px; height: 23px;" align="center">  数　　量</td>                                  
                        <td style="height: 23px">
                                <asp:TextBox ID="tbNum"  Width="72px" runat="server"   onKeyPress="JHshNumberText()" CssClass="input_text"></asp:TextBox>
                        </td>

 <td style="height:23px; width: 56px;" align="center">金　　额</td>
                        <td style="height:23px" onKeyPress="JHshNumberText()">
                                <asp:TextBox ID="tbAmount" Width="70px" runat="server" CssClass="input_text"></asp:TextBox>
                        </td>

function JHshNumberText()
      {
        if ( !(((window.event.keyCode >= 48) && (window.event.keyCode <= 57)) 
        || (window.event.keyCode == 13) || (window.event.keyCode == 46) 
        || (window.event.keyCode == 45)))
        {
        window.event.keyCode = 0 ;
        }
        }

this.tbPrice.Attributes.Add("onblur", "javascript:getSum();");
          this.tbNum.Attributes.Add("onblur", "javascript:getSum();");

  function getSum()
         {         
             var price,num;
             price=0;
             num=0;
             if(document.form1.tbPrice.value!="")
             {
                 price=document.form1.tbPrice.value;
             }
             if(document.form1.tbNum.value!="")
             {
                 num=document.form1.tbNum.value;
             }
             document.form1.tbAmount.value=cheng(num*price,2);
         }  
        function  cheng(num,n)  
        {
               var  dd=1;  
               var  tempnum;  
               for(i=0;i<n;i++)  
               {  
                    dd*=10;  
               }  
               tempnum=num*dd;  
               tempnum=Math.round(tempnum);  
               return (tempnum/dd);  
        }