It originally did always choose the lowest cost. Most people presumed it was intentional, since nobody likes losing to the a boss randomly summoning their 8/8 on turn 1. But that was changed last week:
For all I know it’s still rigged, just less severely. Maybe it prefers small minions for the first few turns and expands its scope later. I would hope something like that is the case, as killing off a run with a turn 1 Lich King would be terrible design.