I didnt read all through to see if this was mentioned, but here is another idea why it costs less to build a truck vs an SUV.
The cabin on an SUV is a huge single piece to have to move around the assembly line, has a ton of parts that have to attach to it, and is probably a pain to get all the interior pieces into.
Whereas a truck, the cab is going to be alot smaller, easier to manipulate on the assembly line, and not as much interior embellisments to contend with. Then you just literally set the bed on the frame, slap in a few bolts, tail light wires, and a fuel hose to the tank, and you're done.
And like said before, another part of it may just be the "luxury" part of it, as they know it may not cost them much more to produce the SUV from a manufacturing standpoint, but market wise it's demand can fetch a higher price.
All hypothetical though.....