RE: Splinterlands Daily Quest Stream Replay - November 21, 2021 - Still not back into diamond ... yet
Well, you earn DEC (the in game currency) with each win once you enter the Bronze 1 league or higher. There's potential appreciation on the value of the cards, too.
Some players rent out their excess cards to others, others play the markets... there's also tournaments which can be entered where players can earn prizes.
So there's a variety of ways to earn, but if you're just playing teh game, you're not losing money by losing a battle, just the opportunity to win the DEC that battle would generate. (and, if its a draw, no one wins!)