module-admin-order-columns/Plugin/Order/View/ItemsColumns.php

<?php
namespace Shopkeeper\AdminOrderColumns\Plugin\Order\View;

class ItemsColumns
{
    /**
     * Modify the columns array to remove tax columns and add cost column
     *
     * @param \Magento\Sales\Block\Adminhtml\Order\View\Items $subject
     * @param array $result
     * @return array
     */
    public function afterGetColumns($subject, $result)
    {
        // Remove tax columns
        unset($result['tax-amount']);
        unset($result['tax-percent']);

        // Add cost column after price
        $newColumns = [];
        foreach ($result as $key => $value) {
            $newColumns[$key] = $value;
            if ($key === 'price') {
                $newColumns['cost'] = 'Cost';
            }
        }

        return $newColumns;
    }
}