I totally agree with you 70chall440, I've been looking into doing the same thing for one of my B bodies. The "kits" are marked up a bit for sure, but one difference I've noticed is that the kits claim the HB unit to be "brand new" Bosch units versus the Rock Auto units are rebuilt Bosch units. So that may account for SOME of the difference in price plus of course the kits are pretty much "bolt on" versus the "some fabrication required" if you build your own system. But, like you, I'm tempted to give it a try with the rebuilt HB unit.
One question that I haven't been able to fully comprehend is why/how does the HB unit create MORE line pressure with the same M/C installed? I've got plenty of "ASSIST" with my factory vacuum booster, just not enough line pressure for the front discs. I can only come up with the possible answer that the HB unit will easily operate a LARGER bore diameter M/C thereby increasing volume AND pressure to the front discs? Whereas now I have to use the smallest bore diameter M/C (15/16") to generate higher PSI to the front which of course decreases fluid volume at the same time.