Nice work! I also built an 80/20 rack for my '08 4Runner. I was lucky enough to get a bunch of 1515 profile extrusions for a song, so I made it out of those.
In case anyone's interested, some details:
1515 = 1.5" x 1.5"
4336, 15 Series, 4-Hole Inside Gusset Corner
Hardware was most cost-effective in the 10-pack including bolt, t-nuts, and washers:
3618, 15 Series, 5/16-18 X 5/8" (.625") Button Head Socket Cap Screw (BHSCS) with Washer and Roll-In T-Nut, Stainless Steel
I ran the deflection calculations, which yielded a certain amount of expected deflection, but that didn't tell me whether the amount of expected deflection was okay or not. I talked to an 80/20 rep and they cited 0.125" as maximum recommended deflection for 1515 profile.
With that, I also did some simple load and deflection testing (notes in screenshot attached). Basically, it handled 250 lbs center point load with the max 1/8" deflection across a 40" span (me standing on the center of a piece spanning a couple 2x4s). That was enough for me to know it's plenty strong for my application. If I was mounting a rooftop tent or something else heavy for long distance/off-road travel, I'd do calculations again more carefully.
I cut some steel brackets out of a thick steel bracket from some kind of projection screen. Cut into 4 quarters, bent to match roof side pitch, drilled, painted, and mounted on some composite-ish blocks I cut to fit in the drip rail channels. Sealed on installation with gasket maker goop.
My cost was just $120 for stainless hardware listed above (I got 60 sets of bolt/washer/t-nut to have some extra), and 8 corner gussets. Could've done with less for the base rack, but nice to have options for mods and mounting other stuff. You'd have to calculate cost of the extrusions to get a full cost.
Note, the black hardware is coated steel, but is not corrosion resistant. Get stainless hardware for exterior/roof rack use.
Note, after less than a year on my roof, the black extrusion coating is getting faded. My truck lives outside, so it's 24/7 winter weather and UV rays. Still working fine, doesn't look bad, and rusty just a concern with aluminum. But I might give them a coat of some wheel paint or something made to withstand extensive exterior exposure.
I've also used some additional extrusions to extend my rack forward for some short distance hauling of light 12' boards. I dig the versatility of the setup.