This is directly from the Game Manual.
Players with at least 60 skill points and a salary no higher than $80 000 that are manually fired or released by teams that have been abandoned (gone bot) are put on the transfer list at ten times their salary, with the maximum possible listing price for all players being $500,000. If a player can't find a team interested in them at that price, they will begrudgingly accept retirement. Any players that are not deemed as top talent in their age group will just retire right away.