Sounds good thanks!
That is what I thought, but I wanted to make sure.
It also depends slightly on the size of your card collection, since packs lose value as you get more cards (you are increasingly likely to get duplicate cards) while the possible dust reward from Arena does not lose value. When you are opening mostly packs with duplicates, 1 rare + 4 common pack is simply worth 40 dust. At that point the chance for a 20-95 dust reward from Arena is very attractive, and can be worth more than a pack.