So when you create a new custom team you have infinite money to play with, but of course you can't keep infinite money forever. What actually happens is that when you join a league, your bank is set to zero when you start your first game. Usually.
It turns out you can manipulate things by buying things (apo, reroll, players) before you join the league, and selling them afterwards. (Actually you can sell extra stuff immediately.) You'll be refunded the TV cost and it will appear in your bank account when you start your first game. (Also buying new players, more rerolls, etc., comes out of that refunded money.)
What this means is that two completely identical teams can start game one in a fresh league, where one team has no bank and the other has 150k or more. Money in your invisible bank above 150k still counts against your TV too.
Finally, I made a 2600TV elf team for laughs and tested putting that into a league too: it set the team's bank to -1. Not zero, which suggests something may have gone wrong there too but I have no idea what.