You need at least six Trapezohedrons to make just one ultimate weapon for each character, and would somehow fathom raising 12,000,000 Gil to buy them. But there is a trick through dismantling if you already own one Trapezohedron. Create one of the following weapons:
* Kain's Lance (made from Dragoon Lance, Shamanic Spear, Punisher, Pandoran Spear or Gae Bolg)
* Nirvana (made from Tigerclaw, Healer's Staff, Belladonna Wand or Mitstilteinn)
Upgrade either Kain's Lance or Nirvana to maximum level (at a cost of approximately 1,500,000 Gil), and then dismantle it.
You'll obtain three Trapezohedrons, tripling your initial investment, and for rather less than the 4,000,000 Gil you'd have paid in a shop. You'll even get a present of 36 Moonblossom Seeds at the same time, worth 216,000 Gil.