I'm in need of a 30" interior door, to be used as a pocket door. Home
Depot is selling pre-primed doors (no predrilled holes) for $19. They
also sell doors for $34 and I can not find much difference. It is
important that the door remains perfectly flat over time, so I am
leaning toward composite construction doors. Any comments about the
$19 door?

You can rely on the salesbeing at Home Depot to tell you the
difference, or you can write down the part numbers and look them up on
the Internet.

Some of the cheap hollow-core doors have cardboard spacers inside. Less
cheap doors have MDF-type spacers. It's likely something like that.

--
Steve Bell
New Life Home Improvement
Arlington, TX USA