--> (.py) Inherit sale order line
class SaleOrderLine(models.Model):
_inherit = 'sale.order.line'

cost = fields.Float()

@api.onchange('product_id')
def onchange_product(self):
self.cost = self.product_id.standard_price

--> (xml) Put that field in view of order line.

<odoo>
<record id="sale_order_inherit_form" model="ir.ui.view">
<field name="name">sale.order.inherit.form</field>
<field name="model">sale.order</field>
<field name="inherit_id" ref="sale.view_order_form" />
<field name="arch" type="xml">
<xpath expr="//page[@name='order_lines']//tree//field[@name='name']" position="after">
<field name="cost"/>
</xpath>
</field>
</record>
</odoo>