In reply to Gaz lord: In my experience the sole determinant of durability is the face fabric.
Oh for the days of 3 layer taslan goretex- robust, heavy, indestructible nylon faced stuff. I still have a 80s one which I use now and again. It's not light.
The membrane inside is vulnerable whether goretex or event. After all its only clingfilm. Cover it with a lightweight bit of nylon and it will be nice and light, yes, but you will have to buy another in a couple of years beacuse it will have worn out. Light is not right unless you are pushing the limits on an 8000m peak or lightweight backpacking. A couple of hundred extra grams in face fabric equals lot more durability.
As to the OPs question, price differential probably has a lot to do with marketing, not quality.