Many many moons ago there were a couple of rooms above a nice coffee shop right in the centre of town that I used to book for colleagues. I've rung them and they still do B&B but I've no idea about costs etc - they used to be fairly basic but very reasonable though (and they're in a fab position!):
Puccinos 1 Bartholomews, BRIGHTON. Tel: 01273 204656
Some more here from googling but again no personal experience I'm afraid...
aquarium guesthouse
Beynon House
Aymer Guest House