if (!empty($sort)) {
    $paginatorProperties['sortby'] = $mSearch2->getSortFields($sort);
    $paginatorProperties['sortdir'] = '';
}

if (!empty($sort)) {
    $sorting = $mSearch2->getSortFields($sort);
    if ($sorting == '`Data`.`price` desc') {
        $sorting = 'IF (`Data`.`price` > 0, 1, 0) desc, `Data`.`price` desc';
    } elseif ($sorting == '`Data`.`price` asc') {
        $sorting = 'IF (`Data`.`price` > 0, 1, 0) desc, `Data`.`price` asc';
    }
    $paginatorProperties['sortby'] = $sorting;
    $paginatorProperties['sortdir'] = '';
}

/assets/components/msearch2/js/web/default.js

/assets/components/msearch2/js/web/custom.js

mse2Config['actionUrl']

'/assets/components/msearch2/actionCustom.php'

/assets/components/msearch2/action.php

/assets/components/msearch2/actionCustom.php

<a href="#" data-sort="ms|price" data-dir="[[+mse2_sort:is=`ms|price:desc`:then=`desc`]]" data-default="desc" class="sort">По цене <span></span></a>

actionCustom.php

$paginatorProperties['sortby'] = !empty($sort)
            ? $mSearch2->getSortFields($sort)
            : '';

$sorting = '';
            if (!empty($sort)) {
            $sorting = $mSearch2->getSortFields($sort);
            if ($sorting == '`Data`.`price` desc') {
                $sorting = 'IF (`Data`.`price` > 0, 1, 0) desc, `Data`.`price` desc';
            } elseif ($sorting == '`Data`.`price` asc') {
                $sorting = 'IF (`Data`.`price` > 0, 1, 0) desc, `Data`.`price` asc';
            }
        }
        $paginatorProperties['sortby'] = $sorting;

//SeoSuite.addKeywords();