I'm going for the stripped down, Outlaw racer look so I've sourced a Porsche 550 Spyder aeroscreen (from Martin & Walker). I think it was about £120 from memory.
Not a great picture as it's still in the packaging, but it's the same size and shape as the Sammio Spyder screen, albeit a fully moulded one that attaches without the need for a separate frame.
I did look at the glass screen with the fancy chromed brass frame and alloy side posts, but, as you say, it's a mighty expensive option with even the rubber seals coming in at a couple of hundred quid.
I've seen one or two come up on ebay but they're never cheap and are often in the States.
Are you planning to go with a folding roof or just a screen? It'd save a lot of cash if you used a perspex deflector.