Disk Brake Conversion Hub question

I have been doing some searching to no avail, but I know the answer is out there. When I comes to converting your electric drum to disk brakes there are a few kits to choose from. But why $500 for a disk hub, backing plates and calipers? Example; when you purchase hubs for your truck you don't pay more than $50 for the hub maybe $30 for a caliper and $20 for a set of pads? Quite the mark up eh?

So, what automotive disk hub will install on a Al-Ko 7000 lb axle? I can't help to think there is a 3/4 ton or 1 ton truck hub assembly that will fit on the spindles.

Any thoughts?