I don't know exactly, but it is a combination of how many players you have sold in the last 14 weeks and how much time the player has been on your team.
I THINK that if you haven't sold any players, then tax will be 80% for a new player.
I THINK that every player you sell after 14 players increases the tax by 1%.
I THINK that every week a player is on your team his tax decreases by 1%.
3% sales tax is the lowest sales tax you can have regardless of how long a player has been on your team.