Something like £2 a search. Follow the "Help" link on their website.
It used to be free or "credits" could be earned by subscribing to some questionnaires. Needless to say, when they started charging I chose not to subscribe.
If you are searching for someone who you know lives on a fairly short road you can make repeated searches using every number on the road until you get the required "hit". You are limited to 10 free searches a day though so the above approach could get rather tedious!
Other free sites worth a try are bt.com and
http://www.postoffice.co.uk/