I’m not sure how up to date this website is, but it does give some ideas as to cost, a lot depends on how far he has to run cables etc, how easy it is to get to etc.
https://www.yourjobcost.co.uk/outdoor-power-socket-cost/
Previous Motability Cars
2006 - 2009 Skoda Superb VR6 2.0tdi
2009 - 2012 Citroen C5 2.0tdi VTR Nav
2012 - 2015 Nissan Qashqai 1.5dci tekna
2015 - 2018 Ford Kuga 2.0tdi Titanium X
2018 - 2021 BMW 220d X drive 2 Series Active Luxury
2021 - 2023 Hyundai Kona Electric Premium SE
2023 - Hyundai Kona Electric Ultimate