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

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

            if (false === $regular_price) {
                return [
                    'has_error' => true,
                    'message'   => __('Regular price can not be empty', 'wc-frontend-manager'),
                ];
            }

            $sale_price = is_numeric($form_data['sale_price']) ? $form_data['sale_price'] : false;

            if (false !== $sale_price && $regular_price <= $sale_price) {
                return [
                    'has_error' => true,
                    'message'   => __('Sale price must be less than regular price', 'wc-frontend-manager'),
                ];
            }

            $effective_price = (false !== $sale_price) ? min($regular_price, $sale_price) : $regular_price;

            if ($effective_price < $minimum_price) {
                return [
                    'has_error' => true,
                    'message'   => sprintf(__('Price can not be less than %1$s%2$s', 'wc-frontend-manager'), get_woocommerce_currency_symbol(), $minimum_price),
                ];
            }
        }
    }
    return $form_data;
}, 10, 2);