Dwarkin
Chapter size is the main factor in pricing a chapter, but not the only one as far as I can tell.
As for what the other factors are, I have no idea.
As for how many FP are used to unlock a chapter, that is directly down to coin cost to unlock that chapter.
Up to 14 coins is 1 FP, then +1 FP for every 10 coins thereafter. (15 = 2FP, 35 = 4FP, etc)
Unfortunately, as the coin cost isn't just the size of the chapter, the price of a chapter in one novel can't be used to judge the chapter price in another novel.
This is just what I have observed myself and is not official.
(Other than the coin to FP stuff. That is taken from an official email to another poster on this forum.)