Xur's upgrades are for people who got the exotics before the DLC was released. It's NOT "pay to unlock perks/upgrades," you still have to put xp into to unlock its perks. The old exotics only have 300 atk/ 30 light max, the upgrades allow them to be upgraded to get a 331 atk/ 36 light max at the cost of a bunch of materials and resetting all its perks.