dont think its going to be a fair few k more than a standard gti,,, will be worth more but not that much,,,, i searched the market extensivelly before i bought my gti and 2 pirellis came up over the month,,, yes they are dearer but when you compare a pirelli against a gti that has leather, xenons, good cond, good mileage i was finding the gti to be dearer but just depends on the spec and condition.
keep an eye on autotrader on a national search and see the ones for sale against the gti, parkers is a good idea but i think you have to pay for a value,,, when i used a comparison site for car insurance - think it was compare the market im sure it had a car valuer built in and gives you the bottom value for your car,, its just a quote and costs nothing but if you can get a reg for the gti same year etc and also a pirelli see what the difference is