Unless master working an exotic armour piece improved the exotic perk in some way, there is no reason why it should cost more to fully upgrade compared to Legendary Gear other than the fact that an artificial time sink was the goal of this design choice. Master working an exotic armour piece gives the Exact. Same. Benefit that master working a legendary gives; increased mod slots (the same amount), and a slight increase on all stats. There is zero difference, so why is there an arbitrary increase on the material cost? Why am I paying more for less? Master working Exotic weapons is different, not only does master working them require a different system than master working legendary weapons, they also get an additional perk or a bonus to their existing perk. The same treatment must be applied to exotic gear to make master working it worthwhile.