I can't find anything in the current EFL rulebook about confidentiality for transfer fees. I suspect there are some very practical reasons though. The introduction of time restricted transfer windows is the most obvious. If a club announces they have received £500,000 for a player and looks to buy a replacement the potential selling clubs will price their man accordingly. Also transfer fees are often quite complex so not always easy to calculate. There can be bonuses for appearance milestones, international caps, etc as well as a signing on fee to the player. As the transfer will involve the new club taking responsibility for wages there is also further financial benefit to the selling club assuming the replacement is on lower wages. Then of course there are sell on fees to factor in. The total amounts paid and received generally emerge in the annual accounts. Although most clubs spread fees paid over the length of the player's contract.