Splinterlands has paid me the equivalent of over AU$300 since the beginning of 2020 just from someone using my affiliate link. I have no idea how much money I have made from leasing or selling cards or from the in-game currency over the last 2 years of gameplay, but it's definitely been one of the few 100% straight profit investments I have ever made.
Gameplay is simple to learn, difficult to master. The winner of each match is determined by the program itself, and your real gameplay is the team selection within various limitations like Splinter (colour / element), mana cost, and match rules change each time.
I'm currently in the Gold III league where I will win 18 free cards this fortnightly season, but I do hope to climb into the Diamond leagues.