From amex's website:
Best Value Guarantee
With the Best Value Guarantee, you'll pay the lowest price available on items purchased with the Card. If you see a covered item advertised in print for less than you paid within 60 days of the purchase date, we'll refund the difference to you, up to $250 per item.
Best Value Guarantee assures the lowest price available on purchases made entirely with an eligible American Express® Card in the U.S. Refunds are limited to $1,000 annually per Card account. We will gladly honor your request for a refund on a savings of $10 or more. For more details or to file a request for a refund, call 1-800-557-8317. Coverage is subject to certain terms, conditions and exclusions.
The only question is what are those terms, limitations, and exclusions...