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Although if you don't do it, the corks can crak which costs hundreds to repair.
Actually, repairing the cork is probably the cheapest part of the whole repair business.
What costs a lot is the fine tuning of an expensive clarinet. ie the way each key responds and how it affects the other keys. This involves careful examination of the distance each key rises above the tone hole, and then how smoothly the holes are opened and closed. This may involve bending the actual keys, changing the strength of the spring and altering the small pieces of cork glued underneath other keys. This is sometimes straightforward but can often drive you nuts.
Changing the tenon cork, on the other hand, is not difficult. Certainly no more difficult than the fiddly airfix models I used to put together on the kitchen table when I was about 7-10. The main difference is not having a Mum behind you who needs the table for dinner and the words "Just wait till your Dad sees that red paint you dropped on the chair!"
You can either buy pieces of tenon cork from Ebay already cut, or find sheets of cork and do it yourself. In any case, you need a very very sharp craft knike, simple wood glue, sandpaper and preferably a metal ruler.
I think I have links to websites somewhere which I can find if you want.
Steve