How much money when you auction a 1921 silver dollar?

Unless it's a Peace design dollar, there's no real gain if you auction it. You can simply sell it directly to a collector or dealer.

A 1921 Peace dollar would have an eagle with folded wings on the back and is quite scarce. The Morgan design also issued that year has an eagle with opened wings and is by far the most common date for Morgans.

Please see the Related Question for more information.