You can get plastic or masonite doors pretty
cheap. I don't know about $40, but probably
not much more. The trouble is that plastic really
looks like junk, with "sandblasted" fake grain, and
masonite doors are very low-profile, making them
also look fake or odd.
I think the last time I bought a wood panel
door at HD it was $80 for a 32x80. Not cheap,
but not very expensive. Though much of the
reason for the low price is that those doors are
also cheap junk, made of glued-up pieces with
a surface veneer of pine. If they get a lot of
humidity they could start to come apart. And
it's not easy to tell which doors are like that
and which are not. I assume I'd need to special
order a name-brand to get a true solid wood
door, but as far as I know the companies don't
distinguish between solid and glued-up.
The present doors, all 16 of them, have all the door-latch hardware, and hinges already fitted. Rehanging 16 totally new doors would probably triple the time and effort involved compared to just putting carefully measured and cut trim on them.