If Xur work the same way that the rng works in the game, he will have several different rolls that will complete change the math behind what you are trying to figure out. 1 would be to see what type of items he sells so that would be a 33.33% chance 1 would be what the items actually are, so depending on how many items of that type are it would be a 50% or 33.33% chance 1 would be the stats on the items ( only on the armor ) there are many different factors to try to think of when doing the math behind Xur lol.