Rugers with the "200th Year" stamp are known as "Liberty Models", made in 1976. They generally command a premium of $75-$100 on NIB guns, somewhat less to zero on used guns depending on condition. The market is still a bit unsettled from Ruger's discontinuance of ROA production a year ago, and Liberty Models are getting a bit scarce. If it's truly excellent condition $500 is not out of line, $600 is a bit above market.