What would be a good price for a contractor to come in and cut out a doorway and hang a prehung door?

From years in construction industry, using sub-contractors, get written quotes ! Three or more contractors.

Use a written list of requirements . . . that must be completed to get paid !

Make sure they all know that you are get other quotes !

We had all bidders attend a meeting at the site to be quoted at the SAME time.

Interior, not much. Exterior a lot more.

One important issue is whether the wall where they are going to put the door is a load bearing wall. That won’t stop them, but it does mean that they need to take some extra steps during the construction to make sure that the load (downward pressure) goes somewhere. They’ll probably put in a 2×12 header above the doorway.

So as with everything, details matter.

$500 sound fair?

@Dutchess_III That’s exactly what I was thinking, 500.

Is this the secret door?

Not quite at that point yet. But working on it @Forever_Free!

I can’t wait to know when the secret door is in!!!!

You bet!