add_filter( 'wcfm_form_custom_validation', function($form_data, $wcfm_screen) {
    $error = [];
    if ('product_manage' == $wcfm_screen) {
        $minimum_price = 1;

        if ( isset($form_data['regular_price']) && isset($form_data['sale_price']) ) {
            $regular_price = $form_data['regular_price'] ? $form_data['regular_price'] : 0;
            $sale_price = $form_data['sale_price'] ? $form_data['sale_price'] : 0;

            if (min($regular_price,$sale_price) < $minimum_price) {
                $error['has_error'] = true;
                $error['message'] = sprintf(__('Price can not be less than %1$s%2$s', 'wc-frontend-manager'), get_woocommerce_currency_symbol(), $minimum_price);
            }
        }
    }
    return !empty($error) ? $error : $form_data;
}, 10, 2 );