At first look Unbound Flourishing is made to serve the many Hydra decklists floating around. But those never made it to the top-tier shelf of Modern, so that's not what players should be looking at.
Instead you may want to bring it into Tron with Walking Ballista or Affinity with Hangarback Walker. Both will grow huge amounts of counters.
But that price still looks too high, and even if Unbound Flourishing finds its way into some of the top-tier decks, it should settle at around $6-7 range.