Sims,
I can't get to 8m2.
If my calcs are correct (and it may well be wrong), a tank with that dimensions would use 4.565 m2 or 4.79 m2 depending on which one of the last two dimensions is the breadth and which the height. And R3,800 means you paid between R793 / m2 and R832 / m2. Taking into consideration the labour transport etc. your glass price could well have been the same as Titus' or more expensive.
Please correct me if I'm wrong as this has gotten me curious.