<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;
}
var summ = 0;
var services = document.getElementsByClassName("right-form-calculator__cost");
for (var i = 0; i < services.length; i++) {
   summ += parseInt( services.item(i).innerHTML );
}
document.getElementsByClassName("right-form-calculator__sum")[0].innerHTML = 'Сумма ' + summ + ' <span>руб</span>';

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.