It's a few years ago (early 80s), but I did a trip to Guernsey and was able to buy spirits at 'Bonded' prices in the town. At that time it meant a bottle of Vodka was £1.00, whereas if I had bought it at 'Duty Free' prices at the airport, same bottle would have been £5.00 (which itself was somewhat cheaper than mainland prices).
Most duty free shops give you the impression that you are not paying any tax and therefore it must be cheaper. I'm wise to that, and as you can usually buy before you leave the airport, I now check out the prices for something I might want and compare it with the local shops. It's often cheaper (in Europe at least) to buy before your return journey in a supermarket.