I just googled the price in U.S.
For example the vintage Alhambra YG onyx 20 motifs necklace:
In US: $15,900
In Paris: 14,000
In UK: £10,400
I would go to where I can get the most VAT refund given that there isn't huge difference on the published fixed price in these 3 places.
Hope this helps
Thank you!