We have seen it valued between £80-120 in the past. Knockando is part of Diageo's Classic Malts Selection and they recommend you refer to price guides for bottles and antiques or auction sites for a quick market value assessment. Some price guides include Kovel's Bottle Price List (published by Kovel's), Bottle Price Guide by U. Cleveland and Antique Trader. Ebay is also another good source, but we did not see what you're looking for on their site. Another source is Martin
Be sure to have outstanding photos of your bottle of Knockando - anyone you send your request to will certainly want great shot(s). Also, feel free to upload an image here on the Forum.