At events, I've been selling Errant book 1, which is a full-colour perfect bound book of 180 pages, covering the first four full chapters plus extras, for £10 GBP (roughly $12 this morning, though it often fluctuates), though my partner keeps pushing me to raise it to £12 or even £15, since a lot of people are charging more like £15-£20 for books of the same length and quality.
For me though, the book's unit cost is only about £4-5 or so, so £10 is already a 100% markup from unit cost. People are telling me I should sell them for more, but that's difficult for me, because I'm like... but the markup is already good, I'm already making a good profit on these!
That said... it's a tricky one, right? Because sometimes, weirdly, charging more makes a thing sell better because it's perceived as more valuable.