I'm no expert but did some digging as I love a good cognac. Did a google and that points you in the direction of 350-400 Euros. Good enough for a cellar find but no more than a good XO from your local store. Prolly has to do with the fact that it's bottled by Hawker, a french bottle would fetch far more.
I believe to see some strange colouration on the inside of the bottle? Than the cognac is most likely spoiled and the bottele has only collector/vintage value and no "old cognac"value. Bottles like this have to be stored upside down, so the fluid has to be on the corc. That corc is not airtight, therefore the content of the bottle is in contact with O2 and CO2, which spoiles it. Having the fluid on the corc, the corc doesn't dry out and the wine of cognac is preserved.
If you want to be sure, I would go to a good wine dealer.