How expensive is it to put in a door?

it depends but you might want to check home depot or lowes - they have installation services. you mentioned thick walls - have u considered pocket doors?

Are you moving electrical? Plumbing? You need to know what's behind the wall.

punch the hole and put a door, not to expensive. to move what's behind the walls will cost more then the installation.

Just punch the hole? No need to wonder if this is a load-bearing wall, or think about installing a header? And the floors, no need to patch those either?

Flarf is right. Installing doors where done did not exist could actually be costly.

Is there an opening now? Are the doors custom? What kind of hinges? pivot hinges are more expensive. You have floor repair, you need to finish off the walls after the demo, you might have to paint the whole wall again

No one wants to give a number. I will. With a lot of latitude to adjust it depending upon answers to above questions, I say $5,000. Plus any fees imposed by building (there will be alteration fee, possible building engineer fee).

not load-bearing, plaster/gyp block, no plumbing/electrical to reroute: 2 or 3 days labor; materials highly variable, depending on door quality, pre-hung or not, molding, hardware etc.

reroute of electrical add $200
reroute of plumbing: dont do it