Whats the most efficient way to do this?

I'm no mathematician so im hoping someone else can tell me the most efficient way to spend my money to accomplish these two goals. I need 12 rare wildcards to finish a brawl deck that i'm building. I thought about purchasing them outright for about 30 bucks. But I am also focused on building my collection of cards and thought that maybe it would make sense for me to spend the 90 bucks or whatever it costs to purchase 90 packs which would help with my collection while still getting rare wildcards. How many rare wildcards can I expect in 90 packs?