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
Dont get your hopes up if there is any other issues with the card, though
A singleton Temur Battle Rage may seem strange, but with Treasure Cruise, Jace, and cantrips it is not difficult to dig through twenty or 30 cards to find it, so I think it is important to always have access to at least one
But my experience so far has made me feel like with just a few more games, I could give it a shot
The card artwork is a particular highlight
Chivalrous Chevalier Can it be a backhanded compliment