let persons = document.getElementsByClassName('counter-block-input')[0],
            restDays = document.getElementsByClassName('counter-block-input')[1],
            place = document.getElementById('select'),
            totalValue = document.getElementById('total'),
            personsSum = 0,
            daysSum = 0,
            total = 0;

            totalValue.innerHTML = 0;

            persons.addEventListener('change', function() {
                personsSum = +this.value;
                total  = (daysSum + personsSum)*4000;
                if (restDays.value === '') {
                    totalValue.innerHTML = 0;
                } else {
                    totalValue.innerHTML = total;
                }
            });

            restDays.addEventListener('change', function() {
                daysSum = +this.value;
                total  = (daysSum + personsSum)*4000;
                if (persons.value === '') {
                    totalValue.innerHTML = 0;
                } else {
                    totalValue.innerHTML = total;
                }
            });

            place.addEventListener('change', function() {
                if (restDays.value === '' || persons.value === '') {
                    totalValue.innerHTML = 0;
                } else {
                    totalValue.innerHTML = total * this.options[this.selectedIndex].value;;
                }
            });





<div class="counter" id="price">
                <div class="counter-title">Рассчитайте стоимость вашего отдыха
                </div>
                <div class="counter-block">
                    <div class="counter-block-option">Количество людей</div>
                    <input type="number" min="1" step="1" class="counter-block-input">
                </div>
                <div class="counter-block">
                    <div class="counter-block-option">На сколько дней</div>
                    <input type="number" min="1" step="1" class="counter-block-input">
                </div>
                <div class="counter-block">
                    <div class="counter-block-option">Выберете базу</div>
                    <select name="place" id="select">
                        <option id="mumbai" value="1">Индия, Мумбай</option>
                        <option id="kerala" value="1.5">Индия, Керала</option>
                        <option id="varanasi" value="1.8">Индия, Варанаси</option>
                    </select>
                </div>
                <div class="counter-total">
                    Общая сумма<br>
                    <span id="total">20456</span>
                </div>
            </div>