As a base, if they offer 50% I ask for the other 50% at least in fees, that used to work for me.
Since my earlier post I have found that making my demands compulsory, they sometimes agree, it's still mucked up my loan system. Even League 2 teams will only take them as fringe players,
EDIT it seems it only happens with lower league teams, Ath Madrid just offered important player, 60% and 150k playing for a player.