a shedload? its dropping a rear subframe and bolting a new one back in, and changing a fuel tank....
I think if you wanted a quattro from the outset it would make sense just to buy one, but if you already own the FWD and have modified it extensively and decide you want to upgrade to quattro, then it makes sense to simply change the rear sumframe, tank and gearbox, than to find £n000 to buy the quattro, transfer all the mods over, and possibly have to get things painted or what have you, then try and sell your old car. compared with maybe a grand tops in parts and a couple of days work.
Similarly, if the FWD was very cheap, then it might actually still be worth buying it and fitting the quattro system, when compared with buying a more expensive quattro...
Unlike the B5 where its simply not a viable option, this shows that on the B6 it is.