Rough Cost for Double Outdoor Socket

Hi Folks,

I was thinking of getting an outdoor double socket installed and am wondering rough costs in today's market. I've attached a rather crude illustration which shows that on the inside of the kitchen, to the left of where you face the door from the garden we have an electrical socket. I presume an electrician could tap into this and place a socket on the outside at the corner somewhere?

I've no DIY skills whatsoever, so go easy on me with the technical talk! I read in previous threads it's a good idea to have an inside switch to isolate power to the outside socket for when you're on holidays etc.