The quick answer is " you get what you pay for".
Used, rebuilt 1950's technology is a lot cheaper than new, 21st century technology.
I'm not going to even try here to give a dissertation on flat slide carb technology, but you'll get more power, better fuel mileage, and it won't wear out like an SU. The rebuilt SU's are actually only partially rebuilt, not all wear items are replaced, they are not "good as new".
The downdraft weber carbs don't work right without a lot of work, and then are probably still may not be as good as the SUs.
As I mentioned somewhere earlier in this thread, I'm not ready to publish figures showing exactly how much the hp and mpg improvements will be on a b20, as I have just not had time to do the comparison tests in a fashion where I can publish accurate numbers rather than just bolting them on and giving you a seat of the pants analysis. I've done the seat of the pants deal and can tell you I really like the resutls. I've got the comparison numbers in the supercharger application. In that application I'm getting fuel mileage as good as with stock SUs and no SC, but with nearly twice the power. I've seen numbers in other SU replacement auto applications. We know the carbs will flow at least 30% more than an HS6, so it will support more power. Just be patient for the rest. It will come in about a month.
I didn't start this thread and my answers weren't meant as a sales pitch.
As to the price, just go to a motorcycle site and price one HSR 45. I'm not charging to make up for development work.

John
V-performance.com