I'm curious, has anyone attempted to undercut this person recently? How do they respond?
A cloth prism takes 538 gil worth of mats (based on NPC vendor prices) plus the cost of the crystals to make. It's probably lower in practice because the markets are flooded with clear prisms from dungeons. If the board is full of listings for 20@1200 then that sounds like no one else cares to be in that market, unless they're actively repricing people out.