Halfords are clearing out any obsolete sat navs that they still have in stock - it will vary from store to store but if you are lucky you might find a bargain! Here are a few to look out for:
AA Navigator - £34.99
Navman ICN 330/530/550/720 - £34.99 each
Garmin NUVI 310 - with bluetooth! £34.99
Garmin NUVI 300 - £49.99
TomTom GO 500/700 - £34.99
Navman N40i - £99.99
Garmin C510 Deluxe - £49.99
There are quite a few more, many stores won't have these in stock but it is worth asking or poping in as you never know!
Please post back with your finds!