let sum = money_list.list
    .filter(data => data.class === "expend")
    .reduce((a, b) => a.price + b.price);

試試看這樣吧，先filter class 為 expend 的資料，再用 reduce 將所有 price 加總

ps 以上沒測過，有問題自行微調

filter介紹
reduce介紹

Html

<input type="button" value="總計(expend)" onclick="calculate('expend')">
<p id="SumValue_expend"></p>
<input type="button" value="總計(deposit)" onclick="calculate('deposit')">
<p id="SumValue_deposit"></p>
<input type="button" value="總計(total)" onclick="calculate('total')">
<p id="SumValue_total"></p>

Javascript

function calculate(classSelect)
{
  var money_list={};
  money_list.name="收支明細表";
  money_list.time="2020/10";
  money_list.list=[
  {name:"薪水",price:45000,time:"10/5",class:"deposit"},
  {name:"房租",price:7500,time:"10/10",class:"expend"},
  {name:"朋友聚餐",price:890,time:"10/13",class:"expend"},
  {name:"水電費",price:780,time:"10/15",class:"expend"},
  {name:"周年慶（化妝品）",price:15700,time:"10/17",class:"expend"},
  {name:"中秋獎金",price:300,time:"10/25",class:"deposit"}
  ];
  //
  tempTotal=0;
  if (classSelect=='total')
  {
    for (i=0; i<money_list.list.length; i++)
    {
      tempTotal = tempTotal + money_list.list[i].price;
    }  
  }
  else
  {
    for (i=0; i<money_list.list.length; i++)
    {
      if (money_list.list[i].class==classSelect)
      {
        tempTotal = tempTotal + money_list.list[i].price;
      }
    }  
  }
  //
  switch (classSelect) {
  case 'expend':
    document.getElementById("SumValue_expend").innerHTML='金額(expend) : '+tempTotal;
    break;
  case 'deposit':
    document.getElementById("SumValue_deposit").innerHTML='金額(deposit) : '+tempTotal;
    break;
  case 'total':
    document.getElementById("SumValue_total").innerHTML='金額(total) : '+tempTotal;
    break;
  }    
}  

Demo