A week ago I ordered samples of greenhouse glazing from the Sundance Supply company. We got them yesterday. I was pleasantly surprised at not only the quality but the price. It looks like I can glaze my greenhouse in the same glazing as the Cheyenne Botanical Gardens (16mm 3-wall clear polycarbonate) for $3.20 a square foot. Not cheap but not as expensive as I expected either. And by using the 3-wall polycarbonate I don't have to double glaze, it will last much longer, keep heat better and resist hail damage much better than the alternatives. Before I was thinking I'd have to glaze in either recycled windows of varying shapes (a nightmare to seal and prone to hail damage) or the plastic/fiberglass corrugated cheap stuff you find at any home building center, used mainly for patios - which I would have had to double glaze with an interior layer of plastic sheeting for heat retention.
Now all I need to do is find the money.