SU's will give you more fuel effieciency for the same power / possibly more power than webbers , webbers have been known to guzzle, i know grassers with SU's on k10s and iv been told they dont use much more thanstandard carb
Price :Cant give exact figure , ehm well there always on ebay , but there cheaper if you had any classic car autojumble shows or sales in your area , iv got one spare but its got autochoke on it, maybe it can be taken of or decommisioned ,
I also can give you an idea for mounting one to a manifold , i had an idea for an adapter for the mani thats standard , ill sketch it tomorrow and post the picture tomorrow , the idea was it would be simple and cheap to make or not too expensive to get made if your not able