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

Answer 1

I'm not sure if you could just go sell it, but Martell Cordon Bleu Cognac is one of the finest brands out there. A 750 mL bottle goes for about $161 from this website:
http://www.sendbourbon.com/id103259list648product.html
It doesn't say what year it is though.

Answer 2

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.