It is especially true for MMPBs (mass market paperbacks). It's a specific term for a specific format of books that are just recently being phased out. You can find more info about this online.
There are tons of price cuts that paperbacks (and especially MMPBs) use that they don't bother with on (most) hardcovers, because if you're getting a hardcover you probably don't care about absolute lowest cost.
And there's stuff much cheaper than MMPB but they're very rare (think phonebooks and old catalogs).