That's a tricky one. I don't purchase a lot, but I fly out often, more domestic but at least 4-5 international trips a year. All of my domestic flights are United, i.e., Terminal 3, adjacent to the Terminal G of International. Every time I fly out / in I always try to make it to the airport store.
Granted I can't buy anything if I'm just flying domestic, but I at least get to browse and then ask them to put items on hold. It's never large purchases because I always bring carry ons whenever I travel internationally, so I can't buy anything that don't fit in my carry on, so it's only things like shoes, silks, some RTWs and in one occasion a small rose gold bracelet.
IMHO I don't think it's so much the $ value as it is the 'relationship' you build with the workers. They're all really lovely and willing to work with you (they would hold stuff for me for weeks because I have to time my purchase with my international trips). The good thing is, you get to save on tax so yay! DM me if you need more info.