What does an account with all cards on the maximum level in Splinterlands earn? If you don't earn those cards yourself, it is possible to rent some of them and compete at the highest level.
I have broken down the last season earnings in this post for you. One season in Splinterlands lasts for 14 days. I am not playing myself but share my cards with 
Let's have a look what he accomplished in the past season and how much Dark Energy Crystals (DEC) and Splintershards (SPS) we got:
He positioned rank 9 in the Champions league - Awesome!
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Ranked Battles
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Each win in a ranked battle rewards you with DEC. The higher your ranking, the more DEC you receive for each win. The amount also depends heavily on how many others are playing on that day and what the price of DEC is right, which affects its inflation and hence how much is paid out. Over the past season we got 12k DEC together.
Total ranked battles rewards: ~12,000 DEC ($115.04)
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Loot Chests
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Chest value: 59 DEC
Value at current 0.0095873 exchange rate: 0.567$
Estimated values based on the current prices of cards listed in the official Splinterlands market.
Source: Splintercards.com Lootchest Tool
Daily Quests
Daily Quest Rewards 18 x 14 Days: 252 Loot Chests
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The average of the season was spent in Champion II and only at the end entered Champion I. The daily quest rewards are 18 loot chests in Champion II and 20 loot chests in Champion I. This leaves me with an average of 14 days x 18 loot chests = 252 loot chests in total.
252 Loot Chests x 59 DEC = 14,868 DEC ($152.54)
Season Rewards
Season Rewards (Champion I): 150 Loot Chests
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At the end of the bi-weekly season we get the season rewards in form of further loot chests, depending on what league we finish in. For Champion I are 150 loot chests rewarded.
150 Loot Chests x 59 DEC = 8,873 DEC ($85.06)
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Ladder Bonus
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Each league has its own ladder and all top positions from Bronze up to Champion are receiving a ladder bonus at the end of the season. For finishing rank 9 in Champions we get 60,000 DEC.
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Season Ladder Bonus (Rank 9 Champion): 60,000 DEC ($575.23)
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Tournament Prizes
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Since the Splintershards have been released, tournament prizes have switched from DEC to SPS and the total value increased massively. There are now multiple tournaments a week between $10,000 and $60,000 prize pools. They require staked SPS in the amount of the prize pool to enter, plus a hefty entry fee.
Below are both the entry fees and the prizes that
Tournament Entry Fees: ~500 SPS ($275)
Tournament Prizes: 1,678 SPS ($923)
Tournament Surplus: ~1,178 SPS ($648)
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SPS Airdrop
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The Splintershards airdrop is continuing and there are 295 days left (down from 365 days). We got a 95,741 DEC from this season in total (taking the average DEC value of the Loot Chests, not the actual value of what was in the chest).
Assuming we just keep holding this DEC until the end of the SPS airdrop, we would get 13 SPS daily and 3,858 SPS in total.
Source: Splintercards.com SPS Airdrop Tool
13 SPS Daily x 295 days = 3,858 SPS ($2,087)
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SPS Staking
If you stake your SPS in-game, you will currently get 76% APR in SPS rewards (tendency sinking, the more SPS is staked), that are claimable every 3 seconds. I have no idea how to do the math on that for the amount of SPS we daily get from this seasons returns, assuming we stake it daily and re-stake the staking rewards for a compounding effect. But it adds a nice chunk on top for sure.
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Splintercards.com Staking Tool
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Chaos Legion Vouchers Airdrop
For staking SPS you will receive vouchers for the pre-sale of the next card edition Chaos Legion. These vouchers are tokens, that can be traded. You will need $4 plus a voucher token for each pack in the pre-sale. Demand is massive and it is speculated that these vouchers will be traded for several Dollars each. If we stake the daily SPS airdrops each day we will get a few of these vouchers as well.
Splintercards.com Voucher Tool
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Conclusion
The amount of income streams in Splinterlands is enormous. You can post about the game on Splintertalk.io and earn HIVE, SPT, LEO and ONEUP (among other tokens) with the upvotes you receive.
Soon there will be SPS rewards for ranked matches, two leagues instead of one for Modern and Wild format with a new and additional DEC inflation, SPS rewards for Guild Brawls plus earn Merits with which you can buy special Gladius packs with Brawl cards, which are account-bound but do have a DEC burn value, Land expansion with SPS rewards and resources...
Actually I stop it here because none of that is realized yet and I think we have seen enough for today about how much you can earn in Splinterlands. ;-)