I actually think it will be far cheaper to purchase your item in Paris if you will be paying in USD. Especially if your card does not have foreign transaction fees. Currently, the Euro to dollar ratio is 1.12 Euros to 1 Dollar. And I've heard that the dollar will be on par with the Euro by the fall.
Plus, you'll have a nice souvenir from the trip. I've also does a small bit of research on US customs duties upon re-entry into the US. Last time I came in from Europe two years ago each person in your party was allowed $800 duty free. After that, customs apply. Jewelry is subject to a 5 to 5.8 % duty. Please note this was a brief web search and I have not confirmed this with a US Customs agent.
I'm probably going to France in the Fall and I'm going to make an Excel grid that will calculate the full US price versus the full price in France. You may be able to use it as a negotiating point with your local Cartier, too.
The spreadsheet columns would work as follows:
Item. US Retail. Sales Tax. Total US Cost. French Retail. Anticipated Customs Fees. Anticipated Credit Card Transaction Fees. Anticipated VAT Refund. Total French Cost
In Euros. Euro to Dollar Conversion. Total French Cost in Dollars U.S. French Cost Differential.
Then you would have a pretty close approximation of the true costs.
I do complex spreadsheets in Excel all the time, so this would not be hard for me to put together.