The thing with on demand is, you have to stick with standard sized. And go trade size instead of special edition or something else. In other words you have to get the size that everyone thinks of when you say paperback. The larger sizes are more expensive to publish and you don't sell as many.
so you go 4.18x6.88, not 8.25x10.75 or larger. The prices are still not exactly standard, it also depends on the number of pages and a few other things Pick cheap. Good luck.