I have a ceiling price that I won't exceed unless it's an omnibus. This is just high enough to include most agency-priced books, but in practice I hardly ever buy any because there's no urgency. They go on my wish list and languish there indefinitely. (A couple of them have gone up in price, which is quite annoying.)
I would buy them when I want to read them, but I have hundreds of other books to choose from and never quite want to read them that much.
I am suspicious of very low prices, if it's not a time-limited sale. I think there is a sweet spot at about two-thirds of the agency rate where I feel something is high quality and good value. Below that I'll want to verify the quality.