In reply to harvmarv:
Probably more expensive since they're a niche item, so not enough are manufactured to get the price per unit down.
I've had a pair of CU Belay Glasses for about 12 years, which are probably the expensive ones you're referring to. They're good value if considered over a long time span - I'd also justify them as being cheaper than physio required for neck issues.
I've heard reports from some people that belay glasses make them dizzy. Not sure if this is with the cheaper or more expensive options, so whichever ones you buy it's probably worth trying them in store.