jquery
$('.quantity-button').on('click', function() {
let summ = $('.nights').val() * $('.summ').data('.nights') + ($('.guests').val() - 1) * $('.summ').data('.guests');
$('.summ').html('$' + summ);
});
let summ = $('.nights').val() * $('.summ').data('.nights') + ($('.guests').val() - 1) * $('.summ').data('.guests');
$('.summ').html('$' + summ);
<div class="quantity">
<input class="nights" type="number" min="1" max="9" step="1" value="5">
</div>
Nights
</div>
</div>
<div class="holder-slider__info-item">
<div class="holder-slider__info-subtitle"># of Guests</div>
<div class="holder-slider__info-title">
<div class="quantity">
<input class="guests" type="number" min="1" max="9" step="1" value="4">
</div>
Guests
</div>
</div>
<div class="holder-slider__info-item">
<div class="holder-slider__info-subtitle">Pricing</div>
<div class="holder-slider__info-title">
<span class="summ" data-nights="55" data-guests="25"></span> USD
<span class="holder-pricing__span">Per Night</span>
</div>
</div>