I buy what I'm interested in reading if I have the money to spare when it comes to my attention. If I'm running a little short at the time, I may buy it later (or not at all if I should happen to forget about it).
So in a sense, I'm always
price sensitive when it comes to books... but I'm sensitive to my
available spare cash, not
the actual price of individual books. If I want
it, and I can afford
it, I buy
it. I'm not going to stick an arbitrary ceiling on my book transactions (and I'm certainly never going to calculate what I spend in a year on books—that way lies madness).