That doesn't make any sense. It's "free", but you have to pay to reserve a slot to not get kicked?
It's not really complicated. He will take people in for free until the alliance is full, but if it is full and people are making offers to get in they will get priority.
Perhaps I phrased that badly... I don't think it's complicated, but that it's so absurd idea to pay 400k for leeching book exp for 5 hours just in case the ally fills up and might get kicked if don't pay up.
Or maybe Leviathan is grossly inflated, here 400k would buy closer to 1 million exp in FC. And as pointed out, 10-30 takes like 90 minutes...