Calculating a total price with group pricing
Removing intermediate variable $webinar_count

$total_price = ($_POST['meeting_type'] == 'w')
             ? (ceil($guests / 10) * $price)
             : ($guests * $price);


OR

if ($_POST['meeting_type'] != 'w') {
    $total_price = $guests * $price;
} else {
    $total_price = ceil($guests / 10) * $price;
}