Hi, actually the trick is in the Aluminium extrusion. It ranges from below 1.2mm to that of bungalow standard of 1.8mm. The thickness of extrusion would determine strength of the window.

I also learnt the hard way. The prices can vary widely depending on a number of factors. You need a good contractor to explain to you on this.

Also watch out for the gasket quality. Whether your window is prone to leaking will be determined by this item.

Good Luck !

FYI - In the end I did not choose LB but went for Elegance Windows.

http://www.elegancewindow.com.my/

Check out the site. A word of caution - They are not cheap either but quality is good.