The costs went into tuning the single-sided magnet to having more sensitivity than the dual magnet... That's not the easiest thing to do. HiFiMan could have easily went with a dual-magnet design to get this (the easier route out), but wanted to lighten the headphones as well. A lot of R&D went into this... Fang is probably still tuning and adding the final touches as we speak... The costs tend not to come from materials, but the R&D that goes into getting the sound just right with that setup and hitting the goals that wouldn't seem possible. I don't think I know of any planar that could be driven on an iPhone alone (no amp) before HiFiMan did it, let a lone a single-magnet model...
You're not just paying for the materials that makeup the headphone, you're paying for the skill and work that went into it. That includes the R&D, and everything else.