Went with Price, Reading Sample and Author from your options. More generally my decision process is as follows.
1. What I feel like reading at the time. Fiction? Non-fiction? A stand alone book or start up a series?
2. Price. I won't pay more than the cheapest print version for an e-book.
3. Is it available in the Kindle store? If so I'll send the sample, give it a read and buy it if I liked it and want to read more.