When I am buying an unknown author the sample pages are hugely important to me, somewhat less so if I have read good reviews from other people who seem to have tastes similar to mine.
0.99: I'll buy it without much thought
1.99: Similar, but the sample pages make a bigger difference
2.99: I have to be pretty convinced I will like it
3.99: For someone I have never read, the sample pages, reviews from other really matter
4.99 and up: I am more likely to spend this (and up) if I have read something else by the same author. Unlikely if not, unless the sample really hooks me.
note: We have a book budget at our house: $50/month, with two of us buying out of that pot. Towards the end of the month, I am more likely to take chances if we have some money left over. We tend to use the library to get us through some months