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
This red instant can act as a hate card for two different archetypes, on top of being a perfect addition for aggressive decks to get through difficult board states
As a result of this free stock pile of resources to tap into many of the most powerful cube cards in some way cost or use life up, and not just black card draw effects
While putting a +1/+1 counter on a land doesn't do anything to the land itself , Black Panther's other ability lets you put those +1/+1 counters onto a creature while gaining you a ton of life and drawing you a card
While most saboteur cards are too slow to see 1v1 Constructed play, they shine in EDH