The first option I tried was CTC's own accomodation at Dibden Manor, unfortunately fully booked for the day I'm going to be there. I don't know how much it costs though, I would imagine a comparable rate to the local B&Bs.
There are quite a few B&B places nearby, typically about £60 or so and some offer a complimentary lift to Dibden Manor. I've booked into one of these for the night.