Patrol Base had some holographics for £45-£50 but I don't know if they're still in stock because it was quite a while ago when they first got them.
I also got the 3x Magnifier behind the holo sight, but had to get a RIS riser so it would align correctly.
here's a pic of how mine looks, but without the RIS riser in this pic. http://i.imgur.com/ZThrzcd.jpg
cheaper ones take 3 or 4 mini batteries like in watches - there are some models that are a bit longer and take regular AAA batteries for a bit more