I have been trying to purchase a grey St. Louis GM and a red Varenne wallet and here is what I have found (I am located in LA)
Barneys Goyard store
St. Louis GM grey 1670 + 9% tax
Varenne wallet red 1125 + 9% tax
London Goyard
St. Louis GM grey 820 pounds
Varenne 550 pounds
+ 120 pounds shipping and customs when it enters the US
So even with shipping there is roughly an $800 difference, but I've heard varying things about customs and that it can be up to 20-25% of the cost of the item which would decrease the difference to maybe $400 or less. Still a nice savings!
From Paris, I am not sure how much it will cost, but I know the prices with VAT are:
St. Louis GM (special colours) 1040 euros
Varenne wallet (special colours) 670 euros
and they actually have you pay for the customs as well so you probably have a better idea of what your savings are. In any case, I've requested prices and hope to hear from the Rue St. Honore store soon!
Happy shopping~