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.
This will cost you 500 € plus a certain amount based on his skills and age. 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 pounds each week, and the best players are nearing the £1m/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 slighlty less skill, due to their enormous wages.