You could also sell or burn cards to generate some DEC to pay for a Wild Pass... I've been selling some duplicates to pay for it... but I'm kind of hoping DEC rises in price so that I could just burn SPS each season to pay for the Wild Pass.
I think I'll definitely get the Mana Warden, but I'm not sure about the promotional cards. Zyriel is down quite a bit in in the secondary markets from her initial sale price so I might just see how these promotional cards fare, and then pick them up on the secondary market when the novelty has worn off. I'm honestly not sure how to think about them.