In rules:

2.2. GRID IRON TAX DEPARTMENT (GITD)
GITD is the sole taxation authority in the Grid-Iron world. The department's goal is to keep the players in their respective teams for at least one whole season before they are transfered out. As a result of this, if the player is at his team for:
- less than 1 week the tax is 23% of the amount the player was sold for
- 1 week, the tax is 22% of the amount the player was sold for
- 2 weeks, the tax is 21% of the amount the player was sold for
- 3 weeks, the tax is 20% of the amount the player was sold for
- 4 weeks, the tax is 19% of the amount the player was sold for
- 5 weeks, the tax is 18% of the amount the player was sold for
- 6 weeks, the tax is 17% of the amount the player was sold for
- 7 weeks, the tax is 16% of the amount the player was sold for
- 8 weeks, the tax is 15% of the amount the player was sold for
- 9 weeks, the tax is 14% of the amount the player was sold for
- 10 weeks, the tax is 13% of the amount the player was sold for
- 11 weeks, the tax is 12% of the amount the player was sold for
- 12 weeks, the tax is 11% of the amount the player was sold for
- 13 weeks, the tax is 10% of the amount the player was sold for
- 14 weeks, the tax is 9% of the amount the player was sold for
- 15 weeks, the tax is 8% of the amount the player was sold for
- 16 weeks, the tax is 7% of the amount the player was sold for
- 17 weeks, the tax is 6% of the amount the player was sold for
- 18 weeks, the tax is 5% of the amount the player was sold for
- 19 weeks, the tax is 4% of the amount the player was sold for
- 20 weeks or more, the tax is 3% of the amount the player was sold for