define('PRICE', '100');

add_filter('gform_pre_render', 'populate_preconference');
function populate_preconference($form) {
    foreach($form['fields'] as &$field) {
        if( $field['cssClass'] == 'inpersonprice') {
            if( is_numeric(getInPersonPrice()) ) {
                $field['basePrice'] = getInPersonPrice();
            }
        }
    }
    return $form
}

function getInPersonPrice() {

    $validate_ccode = isValidCCode();
    $minus_fifty = 0;

    if($validate_ccode) {
        $minus_fifty = 50;
    }

    return PRICE - 50;
}