It's worth what you will pay for it. But you can also check auction archives on TeleTrade, completed auctions on eBay or try Numismedia Fair Market Value prices here: [FONT=VERDANA,HELVETICA,ARIAL]2006 P Silver Dollar 20th Anniversary Reverse Proof[/FONT]
I might add the $222 price is Numismedia retail. their wholesale price is $185 for the same coin. They also list NGC and PCGS site unseen dealer trading, at that grade, as $165. Exactly what you paid.