I have a bottle of Martell Cordon Bleu 0.7L, around 20 years old. How much is it worth now?

its worth however much you can sell it for. Since liquor doesn't age anymore once it is taken out of the barrel and put in a bottle, it should taste the same as a new bottle, as long as it has been kept away from light. Unless it is in a special collector's bottle, it is worth the same amount as a new bottle.