If its 1 crystal for 90 cloth then your assuming the price for cloth would be only just over a 1 gold a piece.
Cloth is more than that now on my server. And with the decrease of dailies. And no 5 mans coming. I dont see a huge on slaught of supply of cloth being available. If cloth stays around 2-4g a piece like it is now on my server than crystals would cost 250-400g to make the way you are describing.