It depends on the set. I'm pretty certain that all rares and ultrarares were printed on the same sheet, so for a set with 10 URs and 40 rares (I think that's what Priests has), you are likely to get an UR from 1 out of every 5 packs. Apostles has 16 URs, and not as many rares, so you are more likely to get UR's from Apostles than other sets.
