It costs exactly the same as ringing a standard landline number - because thats all it is.
You can choose from a few different area codes, the big cities mainly Birmingham, London, Manchester etc and so the person ringing you won't know that they are ringing Skype. If someone has a special call package e.g. Telewest TalkUnlimited and BT's 5.5p/hour at evenings and weekends then skype numbers will be included in this as well.
You have to pay for a SkypeIn number however, it costs €10 (£7) / 3 months or €30 (£20) / year and comes with free voicemail also.