Or- "Damn, I hate speaking with authority and turning out to be wrong"
This just showed up in my comments overnight, and is most likely right.
"Not that I'm all that much into armour (yet) - but I just read that the earlier type of Bascinet did not have a visor, and was worn UNDER the great helm. Once you got knocked off your horse in a joust, you could throw the great helm away and still wear a helmet with less impaired vision. Could EGG have meant that type of Bascinet? It would explain the AD&D pricing a bit better. I imagine a fitting hinged visor is not easy to make - and is the hardest part of the later bascinet. But if there is none... it might be cheaper and easier to make. http://en.wikipedia.org/wiki/Bascinet
By Jaap de Goede on Helmets at 3:10 AM"
For the most part anyway, the hinged visor certainly isn't child's play; but the multitude of curved surfaces of the later Bascinet and it's more fitted form are probably still the reason why they are so darned expensive.