The value of a player is determined by their rating and their age. The higher the rating of a player, the higher his value. His value also diminishes as he grows older.
Once a player decides to retire, his value will be only half of that as another player with the same age and rating. So if you are looking to sell an old player, you are better off doing it before he decides to retire.
You have to pay your players their wages every round. Wages of a player are determined by their rating and experience. The higher a player's rating or experience, the higher the wages you have to pay him.
The wages of a player are met by the team that he currently plays in. If one of your players is on loan at another club, the other club pays his wages. Similarly, if you have loaned a player from another club, his wages will be added to your team's wage bill every round.