In reply to Si Withington:
I have some Pirate ones that were around that price and are excellent quality.
But I don't like using them much. The problem for me is that, while looking at the leader is much easier, keeping an eye on your belay device and the amount of slack in the rope is very much harder. I really wish someone would produce a modified design such that you actively have to look up a little to see through the prism to the leader, yet you can still look down as you would if you weren't wearing the glasses. Sort of like bi-focals, or actually just the existing belay classes with an extra bit that makes them sit higher on your nose! I haven't tried it, but I suspect a bit of blu-tac between the bridge of the glasses and your nose would work just fine!