Hard to value, as you say they don't come up for sale very often, and very seldom in that condition or as complete.
It would sell best to a collector ( obviously ) so I don't think that old sales price was too far off the mark.
In the right place I can see that selling for £500 + in that condition.
EBay would be the place to sell given the worldwide audience, or maybe thewatchsite forum for the Seiko heads.