I have a set of timber centreboards as spares, originally off a mossie, they are very well made and fit the cases perfectly, this maybe a cheap consideration, to make a spare one out of timber/glass.

The thing is with boards and blades, if people buy a new set they hang on to the old ones for spares, and when people do sell them they are usually nearly brand new for some reason and have the price to suit, it may be that they sell their boat with the old set and get some extra cash by selling the new ones seperate.