I'm not really sure it being a quattro version really adds that much value, there is more to go wrong, more weight and it's more costly to run with the MPG being significantly lower and tax higher. That being said people do love a quattro!
You can always get a baseline by looking on eBay and checking the sold listings. One sold on their recently for £720 with rough bodywork and 205k miles
Equally there are two other listings, one under a grand while the other is at £1900.
Ultimately they are simply worth what someone is willing to pay for one. If I was sticking with the stock power level and lived in an urban area I'd stick with a FWD model, whereas if I planned on tuning the car or lived out in the countryside or somewhere the weather was adverse I'd definitely pick the Quattro and stick some good all weather tyres on it.