Mono green stompy is likely to be one of the cheaper competitive decks. Tyrant is expensive, sure, but not needing to run shocklands will make monocolour a lot more budget than any multicolour combination. Of the monocolour options, green is already established as having a reasonable number of playable cards (Steel Leaf Champion, Ghalta, Tyrant, Llanowar Elves, etc) while the other monocolour deck with any prevalence right now (red) is losing a lot of its powerful cards to rotation when GRN comes out.