Fire Sages

The keyword here does one specific thing: it hands you a red mana every time this swings, but only for the duration of that combat. That expiration clause is what makes the design coherent. You cannot bank the mana toward next turn or hold it up for interaction; you either fold it back into the attack that produced it or watch it evaporate. The natural sink is the counter ability, and the arithmetic is the whole balance point: one red covers only a fraction of the price, so a real +1/+1 still asks for a meaningful outlay on top of the freebie. What you get is a 2/2 that turns into a slow, self-financing threat, growing a little each combat you can afford to feed it. This resolves a familiar problem for cheap red creatures: attackers stop mattering by the midgame unless they scale, and most scaling costs a card or a dedicated pump spell. Here the growth is stapled on and paid for partly by the act of attacking, which is exactly what the creature wanted to do anyway. The catch keeps it honest: every mana routed into its toughness is mana pulled away from developing your other threats, so the sink only pays off once you are already committed to swinging, and swinging repeatedly, into whatever removal your opponent has left.