Popped into my local Morrisons last night and noticed that they had Saw 3 for £6.99 on BD. Don't know how good it is - I've only ever seen the first one!
I was in Morrisons yesterday, and they have a number of Blu-rays for under £12, including some of the other Saw movies (which all seem to be at different prices). Unfortunately, there was nothing I wanted!