quite a lot,including the light,about �120. do it yourself,its quite easy,it dosent have to go to the mains,put a plug on the end and plug it in a socket,like a bedside light.
fix the light to the wall outside,drill a hole for the wire to put through to the inside,make sure its long enough to reach your nearest socket,put a 3 pin plug on the end and hey presto,but make sure the wire you buy is suitable for the job,go to b&q and ask them what light and wire is suitable.
If you do it as NTG suggests, use 3-core flex, same as you would for a bedside light for a 2A or 5A fuse in the 13A plug. You shouldn't use Twin & Earth cable - which is for permanent electrical installations. The lights themselves cost about �20 each. Buy a decent one - say IQD brand.
Well i don't know how much security light cost but for security reason i will suggest you to use video door phone. It is the cheapest solution to secure the house from stranger. It is reliable solution.