If Mk4 Anni's are much to go by, just do a comparrison between that and a standard Mk4 GTi.
There is a big markup, and i do agree with you dave, a trader will just look at it as a bigger money maker, as you can garuntee, when he gets it back to his Forecourt, it will have a huge markup due to demand, exactly why Vw dealerships are having bidding wars to try and land a 5 door version , as its more popular and relatively easier to shift.
My dealership is a prime example, has 3 Gti's, one red standard 3 door, one Black standard 3door, 07 plates, and a white one, (5 door Man) (an Ed30) with a purchaser already lined up. (PeterCooper Hedge End).
only had it less then a week. Red ones been there for a good 3 months!.
If your lucky enough to have say someone from an Ethusiasts Website (Like say Gti_mad)who is willing to spend the money to get the right spec car.Then as a seller, you will recoup more cash way above what a dealer will try and offer you bottom price.