hi there
it is quite easy to install a fan to cut the hole you will need a drill and core bit that will drill your hole at 115mm, fit a pipe approw 340mm long seal it with foam or silicone, fit your vent cover on the outside now the fan can be powered from you light (ceiling rose) but it needs an isolater switch just above the fan this will make the fan work when the light is on, or you could bring a live feed from the light and bring it to the isolater switch at switch level approx 1200mm of floor so you can power it on and off with ease now for cost it depends on where you want to hide the cables by cutting the wall or run the cables in trunking on the surface I would think you could get this done surface mounted for about €200 to €250 but a lot more to chase the walls as a sparky would not do the patching or chasing it is hard to get them to do anything. Best of luck