It's impossible to give any kind of an accurate figure without seeing a job like this. Most electricians hate chasing walls (except in a new build) and would prefer to run wires on the surface. (the plastering, filling etc is not a job for the spark). Slab walls can be equally as tricky, it's rarely easy to fish a cable inside a finished slab wall.
Best thing you can do is ask friends and family for recommendations and call at least three of the names you get to come around and price the job for you.
On a new build 7 extra sockets would set you back about €350 or so.