Earnings ramp up near to exponentially as you move up the ranks, from bronze to silver to gold, etc. As an example, I was making between the teens and twenties in DEC (if memory serves) per battle in the silver league, which amounts to between 10 and 20 cents at the current DEC rate, and I'm now making between 40 and 60 DEC just one league above that, at gold, putting each win around half of a dollar.
I haven't been to diamond since the new change to the reward system, but I'd imagine the next step up is equally as impactful on the payout, if not higher, and this is without even considering the better daily reward chests, which quickly ramp up from only a few chests per day to ten and well beyond, and the far more attractive "season end" reward chests, that number in the hundreds at the Champion league.
Add in the possibility, and I'd argue likelihood, that DEC/ SPS go up in value over time, at least in the short to intermediate term future, and you have yourself a recipe for a rising number of users and even stronger competition for the rewards over time, which seems to be what is happening if you look at the current growth of new sign-ups, starter-pack purchases, etc. More people/ competition should, at least in theory, drive up the value of the Splinterland's economy (DEC and SPS).
Of course, even at the present, it requires quite a bit of strategical skill, if not a very expensive deck of cards, likely both, to be able to grind out earnings in those top tier leagues (Diamond and Champion, specifically), which may be a turn off for some, perhaps even most, people, but certainly a strong motivator to improve one's skill/ deck for others.
RE: A Look At The DEC Inflation | The Splinterlands Reward Token