pos/pos_margin/views/view_pos_order.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
Copyright (C) 2017 - Today: GRAP (http://www.grap.coop)
@author Sylvain LE GAL (https://twitter.com/legalsylvain)
License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->

<odoo>

    <record id="view_pos_order_form" model="ir.ui.view">
        <field name="model">pos.order</field>
        <field name="inherit_id" ref="point_of_sale.view_pos_pos_form"/>
        <field name="arch" type="xml">
            <xpath expr="//group[@name='order_total']" position="after">
                <group name="margin">
                    <field name="margin" widget='monetary'/>
                </group>
            </xpath>
            <xpath expr="//field[@name='lines']/tree/field[@name='price_unit']" position="after">
                <field name="purchase_price"/>
            </xpath>
        </field>
    </record>

</odoo>