I've been working on some 15mm resin walls that I picked up at Rampage earlier in the year. Actually I had the short sections (90mms) from an even earlier show and bought the longer walls (170mm) at this years show. They came unpainted and unbased but were pretty cheap. I think the whole lot cost me less than £5:00.
The finished walls (including gates) come to about 1.7m of wall and should be very useful on the game table.