I agree with you that in most cases you get what you pay for..
All we're saying, is that nVidia/ATi are VERY specific about the way ASUS/GeCube/XFX etc build the cards, they are forced to use the exact same template model that nVidia/ATi developed. (Things like cooling and the stickers can be customized though)
So basically : If you buy an Sapphire/POV/EVGA/whatever card, the GPU's have all been made in the exact same factory, and the boards are made to the exact specifications, you basically buy the same thing.
Why do you pay more for certain cards?
Mostly this is due to companies putting different warranties on their products.