Another suggestion is to set it at the same amount as the salary.
how about if a free agent is listed as $500k and he doesn't get bought, after his auction ends he gets re-listed for half the price but the re-listing can happen maximum 2 times. i.e 1st listing at $500k, 2nd listing at $250k then the 3rd listing goes at $125k if neither of these work then the player retires.