Doing an advanced search and looking at completed listings to see what price they went for, the lowest is £40 and they range upto £80+..
There is always a risk buying second hand, some people are really gentle with their kit and it'd be a good buy, others abuse them, you just can't tell on eBay..
I think CEX sell them for £60 and I believe they come with a 12 month warranty even though they are second hand, so that might be less risky..