you can buy all types of webers from ebay, and yes theyre quite expensive but worth it i reckon, sound mad, more power, and i believe you get the "feeling" in a weber though the throttle etc., more than you will ever get with anything else... My opinion.
A twin sidedraft manifold new might be $250, and a pair of webers might be found from 600 to 1500... 600 needing to be rebuilt of coarse.
Dont forget if you are to get a weber or two, if its not new then generally it will need to have acarby kit put through it/freshen up. Or if its off of another car or larger/smaller engine, all of the jetting/ choking and other bits need to be done!!
Plus a dyno tune aswell which might put you out aorund the 500 mark, (from memory from what people told me, my local mechanic i have my maching work done doesnt get change from 600 when he gets his single downdraught tuned on his rotary....)
But the weber sound and 'funness' is like nothing else, i love mine!