Following comment apply to somewith with a PPL(A) only: There is no minimum hours that must have been flown in order to begin the training. However for the rating to be issued, you need to have flown 50 hours X-Country and have a Night rating.
The full details are explained in CAA GID 15. See
http://www.caa.co.uk/docs/175/srg_fcl_gid15.pdf
As to cost, £12K would appear to be the upper limit for the cost of the flying involved (but not if the requried exams are to be included), but it is a good budgetary figure, especially for someone with low'ish total hours, and/or little instrument flight appreciation, eg. no IMC rating.
(But Evo7 beat me to it with the same info. "Beat me to it" geddit?)