I might use the quick frame stuff for interior use, but I don't know if I'd trust it for a roof rack.
I think it can be cost effective if you keep things simple, and reduce the number of hardware pieces to the minimum. That's what actually gets expensive - depending on the design, the extrusions themselves may be only half your cost.
This is the rack that I built for the X5 - I needed something half-width to work with the sky box and that would also fit my hardigg cases, so for me the 80/20 stuff made a lot of sense. Still, I built it out of 1x3 pieces which are far stronger than I needed. Structurally, 1x1's would have been fine, though the 1x3's do give me secure sides to keep the boxes in place.