So do I have a rare 71 duster 340?

i would agree with 22k for your standard driver resto, maybe a bit more. Very few 71 demon/dusters get in the 28/30K area, the ones that do are usually concourse restos or very high option ones like 4-speed, black paint/orange interior type cars with all the correct stock engine parts. a big plus on yours is the buckets/console.