Footballer Contract Extension
0. If you wish to be able to sell your footballer, never extend him beyond Age 34!!! You cannot sell footballer over age 34 AT ALL. Also, the (specialisation excluded) sale price for a age-33 player is less than half of his price at age 32, so for the same reason, try to avoid extending footballers beyond age 33 as well.
1. (In a certain difficulty level,) Contract extension price is related only to the level of the player and nothing else.
2. In Hard difficulty the extension price of a footballer is exactly 1.5 x the extension price of a footballer of the same level in Normal difficulty. In Easy difficulty it's 0.6 x the price in Normal difficulty.
3. At any footballer level, the price for extending the contract for 2 years is 2.3 times that of extending for 1 year, meaning that the money you pay a footballer for his second year in a 2 (or 3)-year contract is 1.3 x the 1-year contract extension price.
4. Similarly, the price for extending the contract for 3 years is 3.9 times that of extending for 1 year, meaning that the money you pay a footballer for his third year in a 3-year contract is 1.6 x the money the 1-year contract extension price.
5. Correlation between the 1-yr extension price and footballer level: It seems that with the exception for level 100, for every 1 level higher, the 1-yr extension price is between 1.02x and 1.4x price for the previous level. This difference peaks between level 20 and 30. For every level above 60, this difference is below 1.1 and decreases until 1.026 for level 99. I am still gathering more points of data for a beautiful line.
6. Based on #4 and #5, you should extend your footballer for 2 years if you think he will have more than 1.3 the current extension price, or about 5 levels higher in 1 year; or extend him for 3 years if you think he will have more than 1.6 the current extension price, or about 10 levels higher in 2 years. Unless you have plans to sell him.
7. Lv 100 footballers have their extension price skyrocket to 2.56x the price of Lv 99 footballers! But their 2/3-yr extension price is still 2.6/3.9x the 1-yr extension price. Always extend your Lv100 footballers by 1 year.
8. Based on #7, it's best to extend your footballers above Lv 95 by 3 years so that you don't need to extend them for a while at Lv 100. If you have Lv 98/99 footballers that have just a few months above 1 year left and the current transfer window has closed, it might be advantageous to put them in the reserve squad so you can extend them by 3 years in the next transfer window at Lv 99 instead of 100.