Depend how far you want to drop it (and how good the original shocks are -- they won't last very long on a lowered car and don't really work properly past 40/50k). Up to about 35mm you can just use springs, after that the cheapest kit out there is around £300.
Price depends on how much your happy to spend really mate, you can do it on the cheap (ie just springs)or you can do it with shortened shocks to match which will be more, and then for a little more you could get shortened adjustable shocks.
As above though anything more than 30mm and you really need shortened shocks of some sort.