<input id="count" class="form-input" type="number" name="count" required="required" placeholder="Количество" min="1" max="999">
<input type="hidden" name="price" value="500">
<select class="form-input form-input--select" name="delivery" required="required">
<option value="150">+150</option>
<option value="250" selected="selected">+250 руб</option>
</select>
Итого: <span id="total"></span>
const $count = document.querySelector('#count');
const $price = document.querySelector('#price');
const $delivery = document.querySelector('#delivery');
const $total = document.querySelector('#total');
const getCalculateValue = () => $total.innerHTML = (Number($count.value) * Number($price.value)) + Number($delivery.value);
getCalculateValue();
$delivery.addEventListener('change', getCalculateValue);
$count.addEventListener('input', getCalculateValue);