Chambers Dictionary (all 24.7MiB of it) is currently available at Fictionwise with a 77% micropay rebate. That takes it from $50 to $11.50, or from $42.50 to $9.77 if you're a fictionwise club member.
http://www.fictionwise.com/eBooks/eBook70285.htm
This discount /isn't/ available at booksonboard - I checked. Although there it is discounted to $43.
Paul