All sensible questions.
There's a $5 discount at the moment if you
blog about the game before buying it. Being me, there will almost certainly be a contest on my forums at some point where winning a free copy is possible, but I wouldn't expect a major sale event until near the end of the year. List price is unlikely to drop for a
long time, unless for some strange reason it got picked up by a major distributor, which is unlikely for a pure VN.
It is a branching story - there's about fifteen endings, give or take depending on what you qualify as a unique ending. There's no extra gameplay, it's all reading and making decisions.
Lengthwise, the demo covers the prologue and chapter 1. A full playthrough is seven chapters and an epilogue. Chapters 1-4 are the same for all the routes, chapters 5-7 are unique for which of the four primary paths you're on. So if you play through the demo you can extrapolate from that how long the full thing will last you.
My writing style is such that you can't see the whole story in a single playthrough. Whichever route you end up on, you're going to be left with some MAJOR questions about the plot that can only be answered by playing other routes, so you're not 'done' with the game after your first ending.
Total text comes to about 120K words.