Depends on the condition of the base.
A couple of weeks ago, I've bought the same seat (550 F) in nearly new condition - no rust at all - for €150 ($210) from a German Honda dealer.
It had been sitting in the shelf since the 70s when he swapped it for a Guliari seat on a new bike he sold back then.
So I was really lucky but I would not spend more than $200 for a used seat in good condition.