Have a look at the swap shop section at
http://forums.mxtrax.co.uk I just had a quick browse and there not much on there in that price range at the moment but ocaisionaly there will be "cheaper" bikes for sale, they do get snapped up quite quickly though.
Second hand mx bike prices have stabilised and in some cases gone up a bit in the last year mainly due to the price hike of new bikes (new yamaha 450f = over 7k), good 250 2 strokes are making good money also as there is a bit of a 2 stroke revival going on due partly to the costs of maintaining 4 strokes and people finding 2 strokes more exiting to ride.
My eldest just bought a new 2011 suzuki 450f which he managed to get knocked down from almost 6.5 to 6k but he races and takes it quite seriously so likes to keep upto date, although even he coughed at the new yam prices when he wanted to replace his 09 yam 450.