Despite the amazing recent growth in ebook sales, most books sold are still pbooks. Each piece of paper in a pbook costs money, and if you use a self-publishing company (as opposed to a printer), you can get really ripped off on paper charges.
Lightning Source is the dominant Print-On-Demand company, producing books for publishers of all types and sizes, including my own Silver Sands Books. I sometimes use CreateSpace (part of Amazon) and its prices are similar.
At Lightning Source one copy of a 300-page paperback will cost $5.35. If you add two pages (one piece of paper) the price goes up by three cents. Pricing-per-page seems very logical to me, but that's not the way some self-publishing companies work.
Here's the wacky price chart from E-BookTime.com: (Despite the company's name, it also produces pbooks.)
The company says it provides "Book publishing that is . . . affordable." A 351-page paperback selling for $20.95 is waaaaay overpriced for most genres. High pricing can make your book noncompetitive.
You want to sell pbooks. If you want people to buy them, the price is important. Choose your printing partner carefully. If you must use a self-pubco, pay attention to the page count, including the pages added by the company.