It depends on what category the book falls into.
If it is one of my go-to authors, I buy it the moment it comes out, regardless of the cost (well; within the universe of mainstream e-book pricing). Cases in point: when the new Lee Child came out a few weeks ago, I bought it the moment it was available. When the new John Sandford came out yesterday, I bought it. I think both were $14.99. But both of those books will stay on my reader until it dies or I get another one, and I will probably read them once every couple years, as I spend a lot of time re-reading my favorite authors.
For a book that is an impulse buy or a curiousity, I definitely consider price. And, as someone else said, I may put off buying it, and then I may forget about it and never buy it. But I have no idea what my "ceiling" would be.