This is true. I do personally believe that a prop made by Weta is itself most likely a masterpiece and worth much more than a self-made one. However, though these props were indeed screen-used, they were not used in a major motion film, which would not render them as valuable to me if they were.
On the contrary. What is also not as well known is that the live action shorts were also to show "how" a halo film would/could possibly look.
The one thing that I find unique about Halo advertisements in general, is that they don't really fall onto using actual game play, and have gone above and beyond what most other game makers do. They have done alot of live action shorts, not to mention that apartment sized diorama used for the "Believe" campaign. I'd still kill to own one of these pieces, especially the Brute Spiker they auctioned off from one of the shorts