So how much is greenhouse safety glass?

My greenhouse is about 18 months old now and glazed with standard horticultural glass, so I've had it long enough now to get a nice crop of green algae growing under the overlaps. 

Having had a close encounter with some seriously nasty shards of glass when I foolishly opened the door on a windy day I thought I'd check out the price of toughened glass in 4X2 rather than 2X2 sizes (this obviously allows doing away with the overlap) and one of the few online prices I could find equates to £37 per sheet (4mm)! 

I know it is made from individual sheets of ordinary float glass which are then heat-treated, and that although 5 times more resistant to impact damage is very prone to shattering if stood on a rough edge (hence handling difficulties, I suppose) but even so, that's a totally unaffordable price as far as I'm concerned, even if replacing one at a time as and when necessary.

Has anyone managed to source it at less than what is around 4 times the price of horticultural glass? 

personally I'd go for plastic if I decided to replace horti glass for safety reasons....