I haven't been in a physical bookstore in so long things may have changed, but trade paperbacks always used to be round number prices: $10.00, $12.00, etc. Historically, mass market paperbacks started at .25, moved up to .50, then .60, then .75, and after that they never to my knowledge were any kind of round number again. They were $1.95 and $5.95 until some publisher decided to cash in on the extra four cents and went to $5.99 or so. I forget which one it was but I remember it happening. That was about thirty years ago.
We're publishing trade paperbacks, not mass market, so our paperbacks ought to be round numbers, too, but somehow we're adding ".99" to the end of every price. Not as classy as one might like, but I've always understood that people often ignore the .99 and thus think the price is lower than it is. That's why it's to our advantage to stick with .99, and to heck with being classy.