I believe the bulk of the 70t samples were made of 2N alumina (that should worth around $ 325/t) and the quantity of 3N and 4N was minimal.

HPA (4N to 6N) is a different world and you don't need that much stuff to make your tests.  To most buyers, consistency of production  will be a key issue and they will try to get many samples over many production days in order to judge on that factor.   

I say, no more free samples, but you can buy on a kg basis.