Magic: The Gathering cards are often expensive because of their rarity, overall popularity, and utility in various competitive metagames
Building Soundwave, Sonic Spy as your Commander can be done, it just needs a strong focus on token Creatures, forsaking the midrange and control aspects that white, blue and black decks are well known for
Challenge promo cards, like Cinderella and Rapunzel, are both playable and artistically stunning
Urborg, Tomb of Yawgmoth makes all of your lands into Swamps, and Coffers turns that into a ton of mana
It is unclear yet how they will affect the game
It all depends on timing, but even if the cost isnt free its comparable to other spells like Dualcaster Mage and Reiterate