It states under SoftWeave armor (War pg 160) that it was designed for nonstandard metasapients and metavariants. I would take this to mean you can upgrade standard armors to suit a naga.
I've always assumed the 150% multiplier to any gear costs accounts for the difficulty in finding the "equivalent" armor in a design made for Pixies, Centaurs, Naga, Sasquachs, etc.
