JSFiddle - React, Tailwind, and code Playground

HTML

<section class="s1 section">
  <div class="container">
    <div class="section-heading">
      <h1 class="heading main-heading">Рассчитайте стоимость вёрстки</h1>
    </div>
    <div class="section-content">
      <form id="form1" class="form content-form calc-form">
        <div class="row form-box">
          <div class="col-lg-6 form-column column1">
            <div class="form-fields">
              <div class="form-fields__heading">
                <h2>Общие параметры:</h2>
              </div>
              <div class="form-fields__items">
                <div class="field-item field-num">
                  <div class="field-item__sign field-num__text"><span>Количество блоков:</span></div>
                  <div class="field-item__input field-num__input">
                    <input type="number" value="1" min="1" max="100" name="countblocks" id="countblocks" class="calcField" />
                  </div>
                </div>
                <div class="field-item field-num">
                  <div class="field-item__sign field-num__text"><span>Количество разрешений экрана:</span></div>
                  <div class="field-item__input field-num__input">
                    <input type="number" value="1" min="1" max="10" name="countresol" id="countresol" class="calcField" />
                  </div>
                </div>
              </div>
            </div>
          </div>
          <div class="col-lg-6 form-column column3">
            <div class="form-fields">
              <div class="form-fields__heading">
                <h2>Дополнительная информация:</h2>
              </div>
            </div>
            <div class="form-fields form-cost">
              <div class="form-fields__heading">
                <h2>Стоимость вёрстки:<span id="result">3000 руб.</span></h2>
              </div>
            </div>
          </div>
        </div>
      </form>
    </div>
  </div>
</section>

JavaScript

function calc() {
	console.log('calc');
  
  let cblocks 	 = document.getElementById('countblocks').value || 1,
  		countresol = document.getElementById('countresol').value || 1,
      sum				 = 2650 + (cblocks * 350) + (countresol * 5),
      result	   = document.getElementById('result');
      
  result.innerHTML = sum + ' руб.';
};

document.querySelectorAll('.calcField').forEach(field => {
	field.addEventListener('input', calc);
});

calc();