Someone made a trainer that maxes out the level on all your characters and gives you functionally unlimited money to buy all the mother boxes you'll ever want. It's super simple to do as well.
1. Start up Injustice 2 and enter Single Fight
2. Pause and alt+tab out of the game and start the trainer
3. Go back into the game and press 0 on your numpad
4. Wait till you hear a voice say "activated", then alt+tab back out of the game and close the trainer.
5. Finish the match and enjoy!
I would also recommend backing up your save because if you get too much currency you'll actually go negative and you'll never be able to earn it back.
https://www.youtube.com/watch?v=1EEUDFuJ_LY&t=186s