I am preparing to sell a student clarinet that I played about six or eight years ago, but the cork is in bad shape. The pads on the keys and everything else are decent, but for a long time I just wrapped yarn around the cork to make it tighter. The buyer will probably have to have new cork put on, so how much will this cost them? I want to charge a fair price but I don’t know how much getting new corks costs.
Thanks! 
US$10 - $75 depending on the repair shop, and how many of the three cork joints need repair. Add $15 - $20 if the mouthpiece needs corking also.
Thanks!
Just take all the keys off and tell them it’s a recorder 
Some recorders have keys.
I’m glad Dave answered since I was about to contribute that I once bought a wooden clarinet for my daughter, from eBay, then spent an equal amount of $ having it cleaned, recorked, oiled, etc, by the repair guy at a final cost substantially above the above quotes. In the end it still wasn’t too great an instrument and she now plays a pricey Buffet that her instructor prodded her Daddy into springing for.
Many places can tell you how much it will cost before they do it. You might want to have it done, and then sell it because you could possibly get more for it. I’ve found that its usually best to go through local repairmen who work in their garages as opposed to large shops and chain businesses.