As with any duty free goods you need to know the comparable prices at your destination. I usually browse the shops with a handful of common items in mind (cigarettes, JW Blue, ect) where I know the destination price just to check the pricing levels. I rarely buy anything these days because I hate the schlep, and the prices are not real bargains.
If you are picking up a unique item and gifting at your destination that is a different story. And sometimes items have a decent GWP (gift with purchase) attached.
SL