From the Best Buy website, "If we lower our price during the return and exchange period (15 days), we will match our lower price, upon request."
Just keep your receipt and head back to the store to get the lower price.
For online purchases, call them at 1-888-BEST BUY (1-888-237-8289) and request your price adjustment.
Since you purchased this well within the compliance period, and the set went on sale well before Black Friday, get on the ball and get that adjustment!