Collector Boosters are the best way to collect variant rares and mythic rares, as well as traditional foil cards, and they're the only place to collect foil-etched and extended-art cards.
There is no fixed ratio between the appearance rate of the regular version of a card and its alternate versions. Instead, about 13% of Set Boosters contain a non-foil rare or mythic rare with an alternate treatment, and about 10% of Streets of New Capenna Draft Boosters contain a non-foil rare or mythic rare with an alternate treatment. Many cards appear with multiple treatments—for such cards, each treatment appears proportionally less often to the number of treatments. This is true of Collector Boosters as well.