Y’know, I found that mask in a bulk LEGO bin at Bricks and Minifigs. As far as other rare masks go, I have several of the 2001 Noble and Great masks in the non-set colors, including a trans-blue Ruru and a Teal Miru. Pretty nice how it works out.
I’m trying to raise money to buy a prototype Tohunga, so pretty much everything is currently for sale.