Hi Sander. I have phoned around a lot in the last two days.
Indeed none of those I spoke to are even willing to even consider giving a price for the repair, so I am as confused as you are about this 220 Euro cost quotation you received from Contax Hamburg.
I have the same experience as you: Zeiss do not repair the Contax G lenses, but forward these straight to Contax Hamburg.
Then I also phoned Contax Hamburg and they were not really friendly and willing to help, but eventually I got a knowledgeable technician on the phone: I can send it or bring it, but whichever way it will take me about 3 weeks before it gets back. Sending from Holland with insurance is a costly affair (some 20 Euro one way).
I phoned NCR in Amsterdam, they have experience with Contax, but not exactly with the kind of problem I have with the 21 mm lens as they have stopped repairing for Plasmans since several years ago and Plasmans have trained a local supplier to repair the Contax equipment with original parts.
For me, living in the eastern part of Holland, Hamburg is about as close as Tilburg in the south of Holland, where Plasmans is located. I have phoned Plasmans for a second time and I must say that they were the friendliest of the bunch and also knowledgeable; although they do not have an emergency repair facility (neither does Hamburg and NCR): they have advised me to bring the camera with the 21 mm lens together since for the adjustment of the lens to the body they may need my G2. I was surprised by this conclusion, but anyhow, if this guarantees the best repair then this is fine.
By the way in Hamburg they have seen several of the G lenses with my type of problem (and probably Sander's problem as well): they state that the main reason for the failure of the lens to lock is the adjustment of the rotating part of the bayonet versus the outer part of the bayonet (there are fragile parts inside which may have been bent or similar). They have, however, also several times found lose parts in the mechanism, blocking the locking mechanism. All in the entire repair cost can vary a lot.
In conclusion I will bring my lens to Plasmans and within a week I will have it back. Sander, you will have to make your own choice, I wish you luck.