<div class="right-form-calculator__column">
  <div class="right-form-calculator__cost km">1600</div>
</div>

<div class="right-form-calculator__row">
  <div class="right-form-calculator__cost service">750</div>
  <div class="right-form-calculator__cost service">1050</div>
  <div class="right-form-calculator__cost service">1100</div>
  <div class="right-form-calculator__cost service">1700</div>
</div>

<div class="right-form-calculator__sum">
  Сумма 3850
  <span>руб</span>
</div>

.right-form-calculator__row{
  margin: 0px 0px 20px 0px;
}

.right-form-calculator__sum{
  color:red;
}

const kmSum = document.querySelector('right-form-calculator__cost.km').innerText;
    const sumService = document.querySelector('.right-form-calculator__cost.service').innerText;
    const mainSum = document.querySelector('.right-form-calculator__sum');

    const total = kmSum + sumService;
    mainSum.innerHTML = total;