edit- you'll probably find that it costs more than you said to keep this kind of criminal locked up as they generally have solitary cells in maximum security prisons, they have to be guarded so that they are not killed by other prisoners, and have to be put on suicide watch, (i.e someone always has to be watching them) if you say that one person can guard them then that's split into three shifts, even if you said that a prison officer gets £20k a year, then that's £60k per year before you consider the cost of their clothes, accommodation meals, any medical care that they may require...
it might cost £30k a year to keep a regular prisoner, but for the really nasty ones, like Ian Huntley or Fred West it's likely to cost considerably more.