You always get taxed on the profit. The way the profit is calculated and the tax is dependent on how you got the player, here are 3 scenarios:
REMEMBER: If profit is negative then you will not get taxed.
Player you had on your team when you started
Here the profit is calculated as profit = Player selling price - Value of player
In this scenarios you will be taxed 20% of the profit.
Player you bought on the transfer market and resold later
Here the profit is calculated as profit = Player selling price - Price paid for player
1. If you sell a player within 5 weeks of buying him: 95% of the profits will be deducted in taxes
2. If you sell a player between 6-13 weeks of buying him: 50% of the profits will be deducted.
3. If you sell a player after having him for 13 weeks: the tax is 20% of the profits.
Copied & Pasted from
http://www.mentaldog.com/mz/