To do that you need to inherit the class pos_details using python inheritance and registered as a parser by inheriting the abstract model report.point_of_sale.report_detailsofsales using odoo inheritance by extention to be able to set the new parser class extension. In the new parser class you need to override the method _pos_sales_details that it's the responsible for build the data used for the sale report detail table that you need to add the new columns, in that method you need to extend the result dict with the new data to the report. like:

from openerp.osv import osv
from openerp.addons.point_of_sale.report.pos_details import pos_details
class pos_details_parser(pos_details):    
    def __init__(self, cr, uid, name, context):
        super(pos_details_parser, self).__init__(cr, uid, name, context=context)
        self.gross_total = 0.0
        self.localcontext.update({
            'pos_sales_details':self._pos_sales_details,
            'getgrosstotal': self._get_gross_total,
        })
        
    def _pos_sales_details(self, form):
        pos_obj = self.pool.get('pos.order')
        user_obj = self.pool.get('res.users')
        data = []
        result = {}
        user_ids = form['user_ids'] or self._get_all_users()
        company_id = user_obj.browse(self.cr, self.uid, self.uid).company_id.id
        pos_ids = pos_obj.search(self.cr, self.uid, [('date_order','>=',form['date_start'] + ' 00:00:00'),('date_order','<=',form['date_end'] + ' 23:59:59'),('user_id','in',user_ids),('state','in',['done','paid','invoiced']),('company_id','=',company_id)])
        for pos in pos_obj.browse(self.cr, self.uid, pos_ids):
            for pol in pos.lines:
                result = {
                    'code': pol.product_id.default_code,
                    'name': pol.product_id.name,
                    'invoice_id': pos.invoice_id.id, 
                    'price_unit': pol.price_unit, 
                    'qty': pol.qty, 
                    'discount': pol.discount, 
                    'total': (pol.price_unit * pol.qty * (1 - (pol.discount) / 100.0)), 
                    'date_order': pos.date_order, 
                    'pos_name': pos.name, 
                    'uom': pol.product_id.uom_id.name,
                    'cost_price': pol.product_id.standard_price,
                    'gross_profit': pol.product_id.list_price - pol.product_id.standard_price,
                    # or
                    #'gross_profit': ((pol.product_id.list_price - pol.product_id.standard_price) * pol.qty * (1 - (pol.discount) / 100.0)),
                }
                data.append(result)
                self.total += result['total']
                self.qty += result['qty']
                self.discount += result['discount']
                self.total += result['total']
                self.gross_total += result['gross_profit']
        if data:
            return data
        else:
            return {}
    
    def _get_gross_total(self):
        return self.gross_total
    
class report_pos_details(osv.AbstractModel):
    _inherit = 'report.point_of_sale.report_detailsofsales'
    _wrapped_report_class = pos_details_parser

With that code in place you need to use it in the report template like:

<template id="report_detailsofsales" inherit_id="point_of_sale.report_detailsofsales">
    <xpath expr="//table[@class='table table-condensed'][1]/thead" position="inside">
        <th class="text-center"><strong>Cost Price</strong></th>
        <th class="text-center"><strong>Gross Profit</strong></th>
    </xpath>
    <xpath expr="//table[@class='table table-condensed'][1]/tbody/tr" position="inside">
        <td class="text-center">
            <span t-esc="formatLang(line_ids['cost_price'], dp='Sale Price')"/>
        </td>
        <td class="text-center">
            <span t-esc="formatLang(line_ids['gross_profit'], dp='Sale Price')"/>
        </td>
    </xpath>
    <xpath expr="//table[@class='table table-condensed'][2]" position="inside">
        <tr>
            <td><strong>Total Gross Profit</strong></td>
            <td class="text-right">
                <strong t-esc="formatLang(getgrosstotal(), dp='Sale Price', currency_obj = res_company.currency_id)"/>
            </td>
        </tr>
    </xpath>
</template>

And now test it all of this, review the gross_profit calculation because I put 2 versions of how to do it depending of what you need to display. I don't test it, this is just an example of the steps that I would do it but probably it will run ok without modifications.
Hope this helps,