Heathrow Airport used to regularly get caught out with duty free prices higher than nearby off licences. It certainly isn’t unusual to find the same product cheaper in town than the airport duty free. I even found Swiss chocolate at Zurich duty free was more expensive than my local supermarket.
First rule of duty free shopping, know the High Street price. There can be some worthwhile savings but you need to do your research.
Actually I find the term “duty free” to be misleading as it implies than a saving will be made as duty isn’t charged. Often there is no saving at all and if there is, the saving seldom amounts to the total duty in the normal price.
I agree with Gargleblaster, I hate being led on a mandatory meandering walk through duty free to get to or from the aircraft.