Hi all
In defence of those high rebuild costs, I presume the carbs were refurbished (if SU did them) rather than rebuilt, things like spindle bushes replaced, dashpot clearences checked, everything calibrated etc etc, so your getting back a "as new" carb. Its worth checking for play in the throttle spindles as air can leak past and weaken the mixture (apparently)
A rebuild kit will have all new gaskets, float valve's and possibly a new float (well the ones I've had in the past have). But then again if thats all you need (and it saves petrol leaking everywhere, including !intothe engine), thats all you need !! and a damn site cheaper