its the fact that its essential that keeps the costs up. either use ya seals and hope for a drop or pay the price. you'll never get a party without it and its precisely that fact that makes ppl pay so much. it's kinda like dispel to a rdm or ballad to a bard
You can't expect to do anything as a NIN without this spell, swapping between Ichi and Ni shadows during a fight can be a lifesaver, so don't even think about hitting 37 without this spell handy.
Due to the three day campaign currently going on as of this writing, NI is flooding onto the auction, but buy but if you need it people, even if you don't have ninja lvl'd yet, this will knock out a huge headache/money problem for you.