From time to time people will show up at this forum simply to find out how much their old bottle is worth and how to sell it. I think this would be a good place to discuss this question in some detail.
The short answer to the question is simply - Your bottle is worth whatever you can convince someone to give you for it.
There is no real market for old American whiskey for several reasons. The first is that it is illegal to sell alcohol without a license. I know, you are saying "They do it on ebay all of the time!" and that is true, but always with the escape clause that they are selling the bottle, not the contents with a wink, wink, nudge, nudge attitude. Even so they are breaking the law and if a government tax collector wants to come after them, they could. In reality, as long as it simply a small sale of only a bottle or two, they probably will not bother to do so. Even so, I would not be suprised at all if they don't have some people keeping track of the number of full bottle sales and go after those who start selling large volumes of bootleg liquor.
The next problem is delivery. It is a nightmare figuring out which states allow alcohol delivery and those that don't. In Kentucky it is illegal to order liquor through the mail. This could mean the loss of the bottle if Kentucky postal inspectors decide to open the package.
So now you have decided that these problems are avoidable and you are going to sell your bottle of bourbon to someone, so how do you determine a price? The best source for the going price can be simply exploring ebay and see what is being sold there. They have a bottle like yours or similar enough that you can base your price upon it. Don't expect to get rich. Most old bottles are going to sell for amounts ranging from $50.00 to maybe $300.00 on ebay.
Another good source are bottle collectors guides. Kovel's Bottle Collecting Guide is a good source for prices on decanters (empty) and also includes other bottles and whiskey paraphanalia (bar trays, shot glasses, etc...). If the decanter is full you could probably ask about twice the Kovel's price.
There are some exceptions to the rules. The biggest is Stitzel-Weller bourbon. This distillery has achieved an almost cult status with bourbon drinkers. Prices for bottles of Stitzel-Weller bourbon will much higher than say a bottle Old Grand Dad from the same time period. Bourbons that are still around today will also claim a higher price than a brand that disappeared 50 years ago - Old Forester will bring a higher price than King even though they are both Brown-Forman products. Older bottles will also bring a higher price than a newer bottle. A pre-prohibition bottle of Old Taylor Bottled-In-Bond will bring two or three times the price of an Old Taylor bottle from the 1930's. The 1930's Old Taylor will bring more than a 1960's bottle and so on.
I hope that this has answered many questions for the readers looking to sell their bottles. I also hope that some of the collectors here will also add their opinions to this post.
"Our people live almost exclusively on whiskey" - E H Taylor, Jr. 25 April 1873