The wages for each player in Hattrick are calculated using the player's skills. As a player gets better, his wage is calculated at the start of each season.
A players main skill is the single most important factor for his salary. The salary will be 500 € plus a certain amount based on his skills and age. When a player reaches 27 years of age, his salary starts to decrease. Players who are playing out of their home country cost their clubs an extra 20%.
With training, it is possible to overtrain a player, so that his wages become astronomically high. Many National Team players have wages of several hundred thousand euros each week, and the best players are over the 1m/euro p/week mark. Many of these are trained by farm teams who operate solely to support one NT player.
Transfer market wise, these players are generally worth less than players with slighty less skill, due to their enormous wages.
For a detailed salary analysis see: Barthons Salary Analysis