A lot of balance and care goes into choosing the cost of a card, more than I can express here, but looking for similar effects on existing Magic cards can give you an idea of a fair costfor example, a ramp spell that puts two lands into play ought to cost 4 mana, per Explosive Vegetation
Love this thing at the kitchen table in Commander loads
Collective Brutality is typically run in decks that utilize graveyard recursion like Abzan Greasefang
If you stick Curiosity on Niv-Mizzet then all you need to do is draw a card (which is something Niv is pretty damn good at) and then you go off, in a flurry of draws and pings
Originally, we only have Arixmethes' replacement effect to apply -- Winding Constrictor can't apply first because there aren't any counters yet
Its an incredible aura, even in decks that dont care about enchantments