A PLUSBUS ticket costs a set amount (Adult: £3.50, Child: £1.75, Railcard holder: £2.30), in addition to your train fare. The daily, weekly and monthly tickets can be purchased from CMS ticket office. Just for June and July, prices start from as little as £2 a day with online offer and children (aged 5-15 years) pay half-fare for a one-day PLUSBUS ticket.
For information on Stations which offer PLUSBUS scheme, see here