That does sound excessive considering the car is only six years old and, as the Touran is a family car, you'd expect that the tailgate would get used a lot, so it should be more robust than this. We'd suggest initially writing to Volkswagen customer services in the UK (you'll find the details on their website) and pointing out the problems you've had and how disappointed you are, particularly as you've had Volkswagens before. They'll no doubt try and fob you off, but if you keep contacting them (writing is good as you can copy the letters you've sent) they may come back with an offer to help pay towards the cost. It's also worth asking the Volkswagen garage what they think, as they can often come out on your side which is useful when dealing with the main Volkswagen customer services dept.