In my 2 weeks of playing and researching about Splinterlands, I have learned that you have few options to get your Return of Investment (ROI).
By playing the game through Rank:
If you played enough of games and assuming you consider all important factors like the Energy Capture Rate, the league you're in and the percentage of winning you get. You can earn a decent amount of DEC. In my experience at Bronze I, I can get 5-10 DEC depending on my ECR. Assuming you will have an average DEC per win of 6 and you have at least 20 wins per day, depending on the value of DEC you can have your ROI in 10-15 days.By buying and selling cards:
If you have good eyes for cards and it's value, and you have budget to buy cards then you can have this setup. You can buy good cards at low value and sell them when their price will go up. An example of this is Mylor Crawling which I bought at around $15, now his value is around $35 - $45 in just two weeks. If you bought a decent amount of that card then you can easily get your ROI plus your income.SPS airdrop and staking
You can have passive income while playing the game with SPS airdrops. Although you need to have decent amount of DEC and power to have good SPS airdrop. But well, if you enjoy the game and played enough, your DEC will be compounded and you can have at least good amount of SPS airdrop. The airdrop you claim can also be staked in addition. This might be the longest way of ROI but definitely a good one.LUCK!
If your lucky enough in your first daily quest, you can get a Legendary Gold Foil from reward cards which cost around $30+. Or, I have seen people getting 10k DEC as reward. Me personally in my 2 weeks of play, I only get 2k DEC one time. But hey, if it's your lucky day, you can have ROI within the day.
As of now, with my 2k DEC rewards, I have my ROI for the Spellbook. I also bought some cards earlier, and their price now are much higher than the price I bought. My SPS airdrop is at 1.5 per day and is compounding.
As much as I think about ROI, I also enjoy the game a lot that I needed to control my self from playing it, good thing they have this ECR as control.