While most on board duty free doesn't offer good deals, duty free stores actually sometimes can have good deals especially for fragrance, as some of these duty free stores actually carry some special packages that are travel exclusives, like it comes with a free watch, free bag, free travel pouch, etc. Those travel exclusives fragrance packages have a smaller size bottles, which come in a lot cheaper than in stores, as those packages in store only come with full size big bottle. I had once got a small Bvlgari white tea that comes with 1oz bottle fragrance, small bottle of shampoo/shower gel, small bottle of lotion, and fragrance towlette, all in a white pouch for $35. There is nothing that cheap in store for Bvlgari at all.

The only onboard duty free that I ever saw deals is on SQ. They have a huge duty free catalog, and for the christmas and chinese new year holidays, they usually have a 10-15% off even you are not an SQ elite. One time I had compared the same thing that sell on SQ is $15 cheaper than CX for the same product, but their price can go up or down also after a few months.