With regards to cost an RCBO is around £25-£35 and a standard (non RCD) outside socket £10-15, add in something for the cost of the cable, wall plugs, screws, silicon to seal around the socket etc and your looking at realistically no more than £50.
Labour rates of course vary from one end of the country to the other but £200 for what is a small job does seem rather steep….
Have you been in touch with council? Ours offers a “handy man” service for the disabled, might be worth a call to see if they do similar – if nothing else they may be able to do it at a more reasonable cost especially if your a council tenant anyway.