A purely random website - Wiltshire Highways, offers this.
Signs
To erect a warning or regulatory sign on a new post costs £150 - £500 dependant on size.
Direction signs on new posts typically cost between £200 - £1,000 dependant on their size.
(If any sign requires external illumination then a further £500 £1,000 can be added to the cost for connection to the electricity supply).