it Differs. Some clubs will not charge a club if the player is very young or not at a great level they are happy to just put him out.
As a ball park figure its usually between 40 and 50 % of the players wages, some clubs might say you have to pay 40% if he plays 60% if he dosnt. It all depends on the 2 clubs doing the deal.
Mahon cant play for another club so for that reason he might choose to stay but he's not really playing here either.
As for Mochrie, Liam Fox is currently assessing if he wants him back but reading P&B some Championship and League of Ireland clubs want him.