I have the Whyte & Mackay 12 Charles & Diana Decanter with Box in perfect condition.
These are becoming more rare as the years tick by. Crazy price ranges have been seen from 98 pounds to 499 pounds. Everything comes down to condition of the bottle and original packaging.
As someone else mentioned earlier the only price that's relevant is the price someone is willing to pay for it. Don't go by retail value. Contact a local auction house or someone like McTears or Bonhams. They both have valuation services.